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How to enforce subtypes/supertypes in Access 2000?
Hi Graham,
Thanks for the tips. I'll re-examine the use of the autonumbers in the two sub-tables. As to the use of table-level constraints, how do I set this up? I mean, I know I can specify that a particular field can't be null. But how do I force a check on the null value of another field (field B) before allowing one field (field A) to be null? Along the same lines, how can I make sure that at least one field must be have a value? I assume that this would ordinarily be achievable at the form level - but you mention table-level constraints. I'm all ears :-) Going off topic a bit, where the form is concerned I've noticed that my database saves data automatically even if I close the form within pressing save on the toolbar. I assume there is some kind of auto-save when you enter data into a form - but this does not always happen. Sometimes it saves, sometimes it doesn't. How do I force a prompt to save every time the form closes? (I located some example code - which I don't have handy - but it does not seem to work). TIA Bob Graham Mandeno wrote: Hi Bob PMFJI :-) In a Jet (Access) database, there is no way to enforce this sort of entity subclassing at the engine level. The only way to do that would be to have two FK fields in tblContacts - one for IndivID and one for OrgID, and have a table-level constraint (validation rule) to specify that they cannot both be Null. Using the structure you have, you can go most of the way there using a BeforeUpdate event procedure on your ContactType control. Something like this (pseudo-code): If ContactType.OldValue isn't null then lookup corresponding record in table corresponding to OldValue If record exists then Heavy warning message about changing contact type If user wishes to continue then delete old related record else cancel = True End If End If End If BTW, I think you are complicating matters by having separate (AutoNumber?) PKs in your Individuals and Organisations tables. I suggest you make ContactID the PK in both those tables. -- Good Luck! Graham Mandeno [Access MVP] Auckland, New Zealand "Bob" wrote in message ... Thanks for your interest Tina, My full table structure is as follows: tblContacts: ContactID (pk) ContactType (fk) Address1 Address2 City State PostCode PostalAddress1 PostalAddress2 PostalCity PostalState PostalPostCode Tel Fax Mob tblIndividuals: IndivID (pk) ContactID (fk) Title FirstName MiddleNames LastName Suffix EmployerID (fk) (links back to tblContacts.ContactID (1:Many)) EmpDirectPhn EmpDirectFax EmpEmail tblOrganisations: OrgID (pk) ContactID (fk) OrgName TradingName IsACompany (yes/no) ACN (Australian Company Number) ABN (Australian Business Number) Website tblContactType ContactTypeID (pk) ContactType ("Indiv" or "Org") As you can see, tblContacts lists the location (ie residential or business) addresses, postal addresses and (residential or business) telecommunication details for all contacts. The table tblIndividuals segregates the individual-specific biographical information together with the individual's work details. The EmployerID links back to the ContactID field in tblContacts because we often end up acting for employees of existing corporate clients or for muliple employees of non-client organisations. I segregate the Organisation details so that I can record details for all businesses (incorporated and unincorporated (ie sole-proprietorships, partnerships, associations, churches etc)) that simply aren't relevant to individuals. It also enables me to set up a separate table (tblOrgContacts) to identify individual contacts for the organisation entities (a 1:Many relationship is established between the two tables based on tblOrganisations.OrgID (pk) and tblOrgContacts.ContactID (fk)). For our purposes, we do not require any contacts to be linked with Individuals as opposed to Organisations. The above tables essentially constitute the whole set of "contacts" for my employer's business; tblContacts is then linked with tblClients which identifies those contacts that are in fact clients: tlbClients: ClientID (pk) (autonumber) ContactID (fk) (related to tblContacts.ContactID) (1:1 relationship) ReferrerID (fk) (related to tblContacts.ContactID) (1:Many relationship) Regards Bob "tina" wrote in message ... my first thought is: do you really need to to separate the individuals records and organizations records into different tables? suggest you post all the fields in each of those two tables so we can review them; perhaps we can help you combine the two tables into one, with the addition of a single field specifying either "individual" or "organization". hth "Bob" wrote in message ... Hi folks, I am creating a client database in MS Access with the following (simplified) table structu tblContacts: ContactID (pk - autonumber) ContactType (fk) (from tblContactTypes) ContactDetails (text) tblIndividuals IndivID (pk - autonumber) ContactID (fk) (from tblContacts) IndivDetails (txt) tblOrganisations OrgID (pk - autonumber) ContactID (fk) (from tblContacts) OrgDetails (txt) tblContactTypes (serves as a lookup table) tblContactTypeID (pk - autonumber) tblContactType (txt - contains values "Indiv" or "Org") There is a 1:1 relationship between the ContactID (pk) in tblContacts and the ContactID (fks) in tblIndividuals and tbleOrganisations. I have created a form in MS Access for entering client details. At the moment, I have two subforms - frmIndiv and frmOrg - which are positioned on my main entry form. The form contains a combo-box from which the user can select "Indiv" or "Org" as the ContactType. Depending on the value in the combo-box, one or other of the two subforms will become visible. At the moment, the user selects - say - "Indiv" as the ContactType and proceeds to enter details for this type of Contact. When this happens, the ContactID for the current record in tblContacts table is mirrored in the ContactID foreign key in the tblIndividuals table. This is what I want. The problem is that once the user is finished (and whilst still in the same record in the tblContacts table), the user can select "Org" from the combo-box and be provided with a empty copy of the sub-form frmOrg. If the user proceeds to enter data on the sub-form, the ContactID foreign key in the frmOrg will also mirror the ContactID in tblContacts. This results in a record in both of my subtype tables (tblIndividuals and tblOrganisations) having a record which points to the same ContactID in the supertype table (tblContacts). How can I prevent this from happening? - ie make sure that each record in the subtype tables points to a record in the supertype table for which no subtype record has already been created? (That's a mouthful - I hope it makes sense). I've seen some references to "check constraints" on the internet which I believe might help achieve my objective. But - so far as I am aware - I can't impose check constraints on fields in Access 2000. (I have seen a suggestion that this might be achieved by using ADO, but no code example was given). Any pointers would be appreciated. Please note, I am a complete novice at this. TIA Bob |
#12
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How to enforce subtypes/supertypes in Access 2000?
Graham,
Nevermind my last post re the table-level constraints. I just re-read yours: BeforeUpdate event - got it. I'm a bit slow today. :-D I'm still interested re the save prompt question though. Regards Bob Bob wrote: Hi Graham, Thanks for the tips. I'll re-examine the use of the autonumbers in the two sub-tables. As to the use of table-level constraints, how do I set this up? I mean, I know I can specify that a particular field can't be null. But how do I force a check on the null value of another field (field B) before allowing one field (field A) to be null? Along the same lines, how can I make sure that at least one field must be have a value? I assume that this would ordinarily be achievable at the form level - but you mention table-level constraints. I'm all ears :-) Going off topic a bit, where the form is concerned I've noticed that my database saves data automatically even if I close the form within pressing save on the toolbar. I assume there is some kind of auto-save when you enter data into a form - but this does not always happen. Sometimes it saves, sometimes it doesn't. How do I force a prompt to save every time the form closes? (I located some example code - which I don't have handy - but it does not seem to work). TIA Bob Graham Mandeno wrote: Hi Bob PMFJI :-) In a Jet (Access) database, there is no way to enforce this sort of entity subclassing at the engine level. The only way to do that would be to have two FK fields in tblContacts - one for IndivID and one for OrgID, and have a table-level constraint (validation rule) to specify that they cannot both be Null. Using the structure you have, you can go most of the way there using a BeforeUpdate event procedure on your ContactType control. Something like this (pseudo-code): If ContactType.OldValue isn't null then lookup corresponding record in table corresponding to OldValue If record exists then Heavy warning message about changing contact type If user wishes to continue then delete old related record else cancel = True End If End If End If BTW, I think you are complicating matters by having separate (AutoNumber?) PKs in your Individuals and Organisations tables. I suggest you make ContactID the PK in both those tables. -- Good Luck! Graham Mandeno [Access MVP] Auckland, New Zealand "Bob" wrote in message ... Thanks for your interest Tina, My full table structure is as follows: tblContacts: ContactID (pk) ContactType (fk) Address1 Address2 City State PostCode PostalAddress1 PostalAddress2 PostalCity PostalState PostalPostCode Tel Fax Mob tblIndividuals: IndivID (pk) ContactID (fk) Title FirstName MiddleNames LastName Suffix EmployerID (fk) (links back to tblContacts.ContactID (1:Many)) EmpDirectPhn EmpDirectFax EmpEmail tblOrganisations: OrgID (pk) ContactID (fk) OrgName TradingName IsACompany (yes/no) ACN (Australian Company Number) ABN (Australian Business Number) Website tblContactType ContactTypeID (pk) ContactType ("Indiv" or "Org") As you can see, tblContacts lists the location (ie residential or business) addresses, postal addresses and (residential or business) telecommunication details for all contacts. The table tblIndividuals segregates the individual-specific biographical information together with the individual's work details. The EmployerID links back to the ContactID field in tblContacts because we often end up acting for employees of existing corporate clients or for muliple employees of non-client organisations. I segregate the Organisation details so that I can record details for all businesses (incorporated and unincorporated (ie sole-proprietorships, partnerships, associations, churches etc)) that simply aren't relevant to individuals. It also enables me to set up a separate table (tblOrgContacts) to identify individual contacts for the organisation entities (a 1:Many relationship is established between the two tables based on tblOrganisations.OrgID (pk) and tblOrgContacts.ContactID (fk)). For our purposes, we do not require any contacts to be linked with Individuals as opposed to Organisations. The above tables essentially constitute the whole set of "contacts" for my employer's business; tblContacts is then linked with tblClients which identifies those contacts that are in fact clients: tlbClients: ClientID (pk) (autonumber) ContactID (fk) (related to tblContacts.ContactID) (1:1 relationship) ReferrerID (fk) (related to tblContacts.ContactID) (1:Many relationship) Regards Bob "tina" wrote in message ... my first thought is: do you really need to to separate the individuals records and organizations records into different tables? suggest you post all the fields in each of those two tables so we can review them; perhaps we can help you combine the two tables into one, with the addition of a single field specifying either "individual" or "organization". hth "Bob" wrote in message ... Hi folks, I am creating a client database in MS Access with the following (simplified) table structu tblContacts: ContactID (pk - autonumber) ContactType (fk) (from tblContactTypes) ContactDetails (text) tblIndividuals IndivID (pk - autonumber) ContactID (fk) (from tblContacts) IndivDetails (txt) tblOrganisations OrgID (pk - autonumber) ContactID (fk) (from tblContacts) OrgDetails (txt) tblContactTypes (serves as a lookup table) tblContactTypeID (pk - autonumber) tblContactType (txt - contains values "Indiv" or "Org") There is a 1:1 relationship between the ContactID (pk) in tblContacts and the ContactID (fks) in tblIndividuals and tbleOrganisations. I have created a form in MS Access for entering client details. At the moment, I have two subforms - frmIndiv and frmOrg - which are positioned on my main entry form. The form contains a combo-box from which the user can select "Indiv" or "Org" as the ContactType. Depending on the value in the combo-box, one or other of the two subforms will become visible. At the moment, the user selects - say - "Indiv" as the ContactType and proceeds to enter details for this type of Contact. When this happens, the ContactID for the current record in tblContacts table is mirrored in the ContactID foreign key in the tblIndividuals table. This is what I want. The problem is that once the user is finished (and whilst still in the same record in the tblContacts table), the user can select "Org" from the combo-box and be provided with a empty copy of the sub-form frmOrg. If the user proceeds to enter data on the sub-form, the ContactID foreign key in the frmOrg will also mirror the ContactID in tblContacts. This results in a record in both of my subtype tables (tblIndividuals and tblOrganisations) having a record which points to the same ContactID in the supertype table (tblContacts). How can I prevent this from happening? - ie make sure that each record in the subtype tables points to a record in the supertype table for which no subtype record has already been created? (That's a mouthful - I hope it makes sense). I've seen some references to "check constraints" on the internet which I believe might help achieve my objective. But - so far as I am aware - I can't impose check constraints on fields in Access 2000. (I have seen a suggestion that this might be achieved by using ADO, but no code example was given). Any pointers would be appreciated. Please note, I am a complete novice at this. TIA Bob |
#13
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How to enforce subtypes/supertypes in Access 2000?
Graham Mandeno wrote: In a Jet (Access) database, there is no way to enforce this sort of entity subclassing at the engine level. The only way to do that would be to have two FK fields in tblContacts - one for IndivID and one for OrgID, and have a table-level constraint (validation rule) to specify that they cannot both be Null. That's got to be a misstatement! The standard approach to implementing such subclass is to have a superclass for both individuals and organizations; this can certainly be implemented in Jet using referential integrity. If there are only two types, each of which needs a base table in the schema, a ContactTypes table is overkill: [I'm avoiding PRIMARY KEY in favour of NOT NULL UNIQUE because clustering on disk is OT; syntax requires ANSI query mode in the Access UI or an OLE DB (e.g. ADO) connection.] CREATE TABLE LegalPersons ( legal_person_ID INTEGER NOT NULL UNIQUE, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type IN ('Indiv', 'org')), UNIQUE (legal_person_type, legal_person_ID) ); CREATE TABLE Individuals ( legal_person_ID INTEGER NOT NULL UNIQUE, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type = 'Indiv'), UNIQUE (legal_person_type, legal_person_ID), FOREIGN KEY (legal_person_type, legal_person_ID) REFERENCES LegalPersons (legal_person_type, legal_person_ID), last_name VARCHAR(35) NOT NULL... ); CREATE TABLE Organizations ( legal_person_ID INTEGER NOT NULL UNIQUE, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type = 'Org'), UNIQUE (legal_person_type, legal_person_ID), FOREIGN KEY (legal_person_type, legal_person_ID) REFERENCES LegalPersons (legal_person_type, legal_person_ID), trading_name NVARCHAR(255) NOT NULL... ); CREATE TABLE Contacts ( legal_person_ID INTEGER NOT NULL, legal_person_type VARCHAR(5) NOT NULL, FOREIGN KEY (legal_person_type, legal_person_ID) REFERENCES LegalPersons (legal_person_type, legal_person_ID), address_line_1 VARCHAR(30) NOT NULL... needs a key ); OK, I know what you are thinking: how to prevent a contact being created for a row in LegalPersons that does not exist in either Individuals or Organizations? Something like this: CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Org', 1, COUNT(*)) FROM Individuals WHERE Individuals.legal_person_ID = Contacts.legal_person_ID) ), CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Indiv', 1, COUNT(*)) FROM Organizations WHERE Organizations.legal_person_ID = Contacts.legal_person_ID) ), .... Even you implied structure (i.e. omitting the LegalPerson table), you *can* implement the foreign keys in Jet: using the same CHECKs as above: CREATE TABLE Individuals ( individual_ID INTEGER NOT NULL UNIQUE, last_name VARCHAR(35) NOT NULL... ); CREATE TABLE Organizations ( organization_ID INTEGER NOT NULL UNIQUE, trading_name NVARCHAR(255) NOT NULL... ); CREATE TABLE Contacts ( legal_person_ID INTEGER NOT NULL, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type IN ('Indiv', 'org')), CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Org', 1, COUNT(*)) FROM Individuals WHERE Individuals.individual_ID = Contacts.legal_person_ID) ), CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Indiv', 1, COUNT(*)) FROM Organizations WHERE Organizations.organization_ID = Contacts.legal_person_ID) ), address_line_1 VARCHAR(30) NOT NULL... needs a key ); I would not recommend 'mixing' identifiers in this way. However, I trust the above is sufficient for you to realize that it is possible to enforce such constraints at the engine level in Jet. Jamie. -- |
#14
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How to enforce subtypes/supertypes in Access 2000?
Hi Jamie,
How exactly do I switch to ANSI mode? Regard Bob "Jamie Collins" wrote in message ups.com... Graham Mandeno wrote: In a Jet (Access) database, there is no way to enforce this sort of entity subclassing at the engine level. The only way to do that would be to have two FK fields in tblContacts - one for IndivID and one for OrgID, and have a table-level constraint (validation rule) to specify that they cannot both be Null. That's got to be a misstatement! The standard approach to implementing such subclass is to have a superclass for both individuals and organizations; this can certainly be implemented in Jet using referential integrity. If there are only two types, each of which needs a base table in the schema, a ContactTypes table is overkill: [I'm avoiding PRIMARY KEY in favour of NOT NULL UNIQUE because clustering on disk is OT; syntax requires ANSI query mode in the Access UI or an OLE DB (e.g. ADO) connection.] CREATE TABLE LegalPersons ( legal_person_ID INTEGER NOT NULL UNIQUE, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type IN ('Indiv', 'org')), UNIQUE (legal_person_type, legal_person_ID) ); CREATE TABLE Individuals ( legal_person_ID INTEGER NOT NULL UNIQUE, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type = 'Indiv'), UNIQUE (legal_person_type, legal_person_ID), FOREIGN KEY (legal_person_type, legal_person_ID) REFERENCES LegalPersons (legal_person_type, legal_person_ID), last_name VARCHAR(35) NOT NULL... ); CREATE TABLE Organizations ( legal_person_ID INTEGER NOT NULL UNIQUE, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type = 'Org'), UNIQUE (legal_person_type, legal_person_ID), FOREIGN KEY (legal_person_type, legal_person_ID) REFERENCES LegalPersons (legal_person_type, legal_person_ID), trading_name NVARCHAR(255) NOT NULL... ); CREATE TABLE Contacts ( legal_person_ID INTEGER NOT NULL, legal_person_type VARCHAR(5) NOT NULL, FOREIGN KEY (legal_person_type, legal_person_ID) REFERENCES LegalPersons (legal_person_type, legal_person_ID), address_line_1 VARCHAR(30) NOT NULL... needs a key ); OK, I know what you are thinking: how to prevent a contact being created for a row in LegalPersons that does not exist in either Individuals or Organizations? Something like this: CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Org', 1, COUNT(*)) FROM Individuals WHERE Individuals.legal_person_ID = Contacts.legal_person_ID) ), CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Indiv', 1, COUNT(*)) FROM Organizations WHERE Organizations.legal_person_ID = Contacts.legal_person_ID) ), ... Even you implied structure (i.e. omitting the LegalPerson table), you *can* implement the foreign keys in Jet: using the same CHECKs as above: CREATE TABLE Individuals ( individual_ID INTEGER NOT NULL UNIQUE, last_name VARCHAR(35) NOT NULL... ); CREATE TABLE Organizations ( organization_ID INTEGER NOT NULL UNIQUE, trading_name NVARCHAR(255) NOT NULL... ); CREATE TABLE Contacts ( legal_person_ID INTEGER NOT NULL, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type IN ('Indiv', 'org')), CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Org', 1, COUNT(*)) FROM Individuals WHERE Individuals.individual_ID = Contacts.legal_person_ID) ), CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Indiv', 1, COUNT(*)) FROM Organizations WHERE Organizations.organization_ID = Contacts.legal_person_ID) ), address_line_1 VARCHAR(30) NOT NULL... needs a key ); I would not recommend 'mixing' identifiers in this way. However, I trust the above is sufficient for you to realize that it is possible to enforce such constraints at the engine level in Jet. Jamie. -- |
#15
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How to enforce subtypes/supertypes in Access 2000?
Hi Jamie
Yes, it was a misstatement - I meant to say "cannot both be NON-Null" :-) Your solution involving duplicating the sub-type in the sub-tables and using a two-field relationship is simple, elegant and brilliant. You say this is "the standard approach" but I can't believe that I have never come across it before, and it is so simple that I'm ashamed I never thought of it myself! One question though - is there any reason not to put the "common" fields in the superclass table? In other words, why not combine the Contacts and LegalPersons tables? -- Thanks! Graham Mandeno [Access MVP] Auckland, New Zealand "Jamie Collins" wrote in message ups.com... Graham Mandeno wrote: In a Jet (Access) database, there is no way to enforce this sort of entity subclassing at the engine level. The only way to do that would be to have two FK fields in tblContacts - one for IndivID and one for OrgID, and have a table-level constraint (validation rule) to specify that they cannot both be Null. That's got to be a misstatement! The standard approach to implementing such subclass is to have a superclass for both individuals and organizations; this can certainly be implemented in Jet using referential integrity. If there are only two types, each of which needs a base table in the schema, a ContactTypes table is overkill: [I'm avoiding PRIMARY KEY in favour of NOT NULL UNIQUE because clustering on disk is OT; syntax requires ANSI query mode in the Access UI or an OLE DB (e.g. ADO) connection.] CREATE TABLE LegalPersons ( legal_person_ID INTEGER NOT NULL UNIQUE, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type IN ('Indiv', 'org')), UNIQUE (legal_person_type, legal_person_ID) ); CREATE TABLE Individuals ( legal_person_ID INTEGER NOT NULL UNIQUE, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type = 'Indiv'), UNIQUE (legal_person_type, legal_person_ID), FOREIGN KEY (legal_person_type, legal_person_ID) REFERENCES LegalPersons (legal_person_type, legal_person_ID), last_name VARCHAR(35) NOT NULL... ); CREATE TABLE Organizations ( legal_person_ID INTEGER NOT NULL UNIQUE, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type = 'Org'), UNIQUE (legal_person_type, legal_person_ID), FOREIGN KEY (legal_person_type, legal_person_ID) REFERENCES LegalPersons (legal_person_type, legal_person_ID), trading_name NVARCHAR(255) NOT NULL... ); CREATE TABLE Contacts ( legal_person_ID INTEGER NOT NULL, legal_person_type VARCHAR(5) NOT NULL, FOREIGN KEY (legal_person_type, legal_person_ID) REFERENCES LegalPersons (legal_person_type, legal_person_ID), address_line_1 VARCHAR(30) NOT NULL... needs a key ); OK, I know what you are thinking: how to prevent a contact being created for a row in LegalPersons that does not exist in either Individuals or Organizations? Something like this: CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Org', 1, COUNT(*)) FROM Individuals WHERE Individuals.legal_person_ID = Contacts.legal_person_ID) ), CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Indiv', 1, COUNT(*)) FROM Organizations WHERE Organizations.legal_person_ID = Contacts.legal_person_ID) ), ... Even you implied structure (i.e. omitting the LegalPerson table), you *can* implement the foreign keys in Jet: using the same CHECKs as above: CREATE TABLE Individuals ( individual_ID INTEGER NOT NULL UNIQUE, last_name VARCHAR(35) NOT NULL... ); CREATE TABLE Organizations ( organization_ID INTEGER NOT NULL UNIQUE, trading_name NVARCHAR(255) NOT NULL... ); CREATE TABLE Contacts ( legal_person_ID INTEGER NOT NULL, legal_person_type VARCHAR(5) NOT NULL, CHECK (legal_person_type IN ('Indiv', 'org')), CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Org', 1, COUNT(*)) FROM Individuals WHERE Individuals.individual_ID = Contacts.legal_person_ID) ), CHECK (1 = (SELECT IIF(Contacts.legal_person_type = 'Indiv', 1, COUNT(*)) FROM Organizations WHERE Organizations.organization_ID = Contacts.legal_person_ID) ), address_line_1 VARCHAR(30) NOT NULL... needs a key ); I would not recommend 'mixing' identifiers in this way. However, I trust the above is sufficient for you to realize that it is possible to enforce such constraints at the engine level in Jet. Jamie. -- |
#16
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How to enforce subtypes/supertypes in Access 2000?
Hi Bob
The solution that Jamie gave is simple and elegant. To translate it into Access table design terms (which might be more familiar to you than ANSI-92 DDL statements!): Add a unique index to tblContacts involving ContactID AND ContactType. Next add a ContactType field to both tblIndividuals and tblOrganisations and for each, set the default value to the corresponding contact type and set the validation rule to =contact type and set required=Yes. (In other words, an individual MUST be an individual and cannot be an organisation, and vice-versa). Now, add a 1:1 relationship with referential integrity between ContactID/ContactType in tblContacts and tblIndividuals, and the same for tblOrganisations. Now the engine will look after the integrity for you. If a record in tblContacts has a matching record in one of the other tables, then the contact record can neither be deleted, nor changed to the other contact type, unless the related subclass record is first deleted. To answer your other questions: 1. A table-level validation rule can be created in the Table Properties window (ViewProperties in design view). For example: ([IndivID] Is Not Null) Xor ([OrgID] Is Not Null) However, I would NOT use this two-field approach for your particular problem. 2. You can force a prompt before saving a record using the form's BeforeUpdate event. Select case MsgBox("Save changes?", vbYesNoCancel) case vbYes ' do nothing case vbNo Cancel = True Me.Undo case vbCancel cancel = true End Select 3. You can set ANSI-92 mode via ToolsOptionsTables/Queries. Use with caution! -- Good Luck! Graham Mandeno [Access MVP] Auckland, New Zealand "Bob" wrote in message oups.com... Hi Graham, Thanks for the tips. I'll re-examine the use of the autonumbers in the two sub-tables. As to the use of table-level constraints, how do I set this up? I mean, I know I can specify that a particular field can't be null. But how do I force a check on the null value of another field (field B) before allowing one field (field A) to be null? Along the same lines, how can I make sure that at least one field must be have a value? I assume that this would ordinarily be achievable at the form level - but you mention table-level constraints. I'm all ears :-) Going off topic a bit, where the form is concerned I've noticed that my database saves data automatically even if I close the form within pressing save on the toolbar. I assume there is some kind of auto-save when you enter data into a form - but this does not always happen. Sometimes it saves, sometimes it doesn't. How do I force a prompt to save every time the form closes? (I located some example code - which I don't have handy - but it does not seem to work). TIA Bob Graham Mandeno wrote: Hi Bob PMFJI :-) In a Jet (Access) database, there is no way to enforce this sort of entity subclassing at the engine level. The only way to do that would be to have two FK fields in tblContacts - one for IndivID and one for OrgID, and have a table-level constraint (validation rule) to specify that they cannot both be Null. Using the structure you have, you can go most of the way there using a BeforeUpdate event procedure on your ContactType control. Something like this (pseudo-code): If ContactType.OldValue isn't null then lookup corresponding record in table corresponding to OldValue If record exists then Heavy warning message about changing contact type If user wishes to continue then delete old related record else cancel = True End If End If End If BTW, I think you are complicating matters by having separate (AutoNumber?) PKs in your Individuals and Organisations tables. I suggest you make ContactID the PK in both those tables. -- Good Luck! Graham Mandeno [Access MVP] Auckland, New Zealand "Bob" wrote in message ... Thanks for your interest Tina, My full table structure is as follows: tblContacts: ContactID (pk) ContactType (fk) Address1 Address2 City State PostCode PostalAddress1 PostalAddress2 PostalCity PostalState PostalPostCode Tel Fax Mob tblIndividuals: IndivID (pk) ContactID (fk) Title FirstName MiddleNames LastName Suffix EmployerID (fk) (links back to tblContacts.ContactID (1:Many)) EmpDirectPhn EmpDirectFax EmpEmail tblOrganisations: OrgID (pk) ContactID (fk) OrgName TradingName IsACompany (yes/no) ACN (Australian Company Number) ABN (Australian Business Number) Website tblContactType ContactTypeID (pk) ContactType ("Indiv" or "Org") As you can see, tblContacts lists the location (ie residential or business) addresses, postal addresses and (residential or business) telecommunication details for all contacts. The table tblIndividuals segregates the individual-specific biographical information together with the individual's work details. The EmployerID links back to the ContactID field in tblContacts because we often end up acting for employees of existing corporate clients or for muliple employees of non-client organisations. I segregate the Organisation details so that I can record details for all businesses (incorporated and unincorporated (ie sole-proprietorships, partnerships, associations, churches etc)) that simply aren't relevant to individuals. It also enables me to set up a separate table (tblOrgContacts) to identify individual contacts for the organisation entities (a 1:Many relationship is established between the two tables based on tblOrganisations.OrgID (pk) and tblOrgContacts.ContactID (fk)). For our purposes, we do not require any contacts to be linked with Individuals as opposed to Organisations. The above tables essentially constitute the whole set of "contacts" for my employer's business; tblContacts is then linked with tblClients which identifies those contacts that are in fact clients: tlbClients: ClientID (pk) (autonumber) ContactID (fk) (related to tblContacts.ContactID) (1:1 relationship) ReferrerID (fk) (related to tblContacts.ContactID) (1:Many relationship) Regards Bob "tina" wrote in message ... my first thought is: do you really need to to separate the individuals records and organizations records into different tables? suggest you post all the fields in each of those two tables so we can review them; perhaps we can help you combine the two tables into one, with the addition of a single field specifying either "individual" or "organization". hth "Bob" wrote in message ... Hi folks, I am creating a client database in MS Access with the following (simplified) table structu tblContacts: ContactID (pk - autonumber) ContactType (fk) (from tblContactTypes) ContactDetails (text) tblIndividuals IndivID (pk - autonumber) ContactID (fk) (from tblContacts) IndivDetails (txt) tblOrganisations OrgID (pk - autonumber) ContactID (fk) (from tblContacts) OrgDetails (txt) tblContactTypes (serves as a lookup table) tblContactTypeID (pk - autonumber) tblContactType (txt - contains values "Indiv" or "Org") There is a 1:1 relationship between the ContactID (pk) in tblContacts and the ContactID (fks) in tblIndividuals and tbleOrganisations. I have created a form in MS Access for entering client details. At the moment, I have two subforms - frmIndiv and frmOrg - which are positioned on my main entry form. The form contains a combo-box from which the user can select "Indiv" or "Org" as the ContactType. Depending on the value in the combo-box, one or other of the two subforms will become visible. At the moment, the user selects - say - "Indiv" as the ContactType and proceeds to enter details for this type of Contact. When this happens, the ContactID for the current record in tblContacts table is mirrored in the ContactID foreign key in the tblIndividuals table. This is what I want. The problem is that once the user is finished (and whilst still in the same record in the tblContacts table), the user can select "Org" from the combo-box and be provided with a empty copy of the sub-form frmOrg. If the user proceeds to enter data on the sub-form, the ContactID foreign key in the frmOrg will also mirror the ContactID in tblContacts. This results in a record in both of my subtype tables (tblIndividuals and tblOrganisations) having a record which points to the same ContactID in the supertype table (tblContacts). How can I prevent this from happening? - ie make sure that each record in the subtype tables points to a record in the supertype table for which no subtype record has already been created? (That's a mouthful - I hope it makes sense). I've seen some references to "check constraints" on the internet which I believe might help achieve my objective. But - so far as I am aware - I can't impose check constraints on fields in Access 2000. (I have seen a suggestion that this might be achieved by using ADO, but no code example was given). Any pointers would be appreciated. Please note, I am a complete novice at this. TIA Bob |
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How to enforce subtypes/supertypes in Access 2000?
Graham Mandeno wrote: duplicating the sub-type in the sub-tables and using a two-field relationship is simple, elegant and brilliant. You say this is "the standard approach" but I can't believe that I have never come across it before, and it is so simple that I'm ashamed I never thought of it myself! One question though - is there any reason not to put the "common" fields in the superclass table? Yes, the shared fields would be best in the superclass table. As I'm sure you are aware, there is distinction between a shared attribute (inheritance) and two attributes sharing a name (coincidence) e.g. patient may have a legal name attribute and a medication may have a legal name attribute but they do have a shared superclass. why not combine the Contacts and LegalPersons tables? It's probably a 1:m relationship e.g. an organization may have more than one contact. Also, an organization may have a contact that is an individual who has a collection of contacts in the context of being a contact for that organization as well as contacts in their own right e.g. work, home (and they may have more than one home), etc. There is certainly potential for the contacts to be a structu each contact type - postal, email, web, telephone, fax, etc - should have probably be in a table of their own, with perhaps shared super classes (e.g. telephone and fax)... this can quickly get out of hand! One issue here is whether all contact types share a superclass; if so, what would the identifier be? There is a similar issue with the proposed LegalPerson superclass: is there a real life identifier shared by organizations and individuals? Jamie. -- |
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How to enforce subtypes/supertypes in Access 2000?
Bob wrote: How exactly do I switch to ANSI mode? About ANSI SQL query mode (MDB) http://office.microsoft.com/en-us/as...704831033.aspx Jamie. -- |
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How to enforce subtypes/supertypes in Access 2000?
Thanks, Jamie.
-- Cheers, Graham "Jamie Collins" wrote in message oups.com... Graham Mandeno wrote: duplicating the sub-type in the sub-tables and using a two-field relationship is simple, elegant and brilliant. You say this is "the standard approach" but I can't believe that I have never come across it before, and it is so simple that I'm ashamed I never thought of it myself! One question though - is there any reason not to put the "common" fields in the superclass table? Yes, the shared fields would be best in the superclass table. As I'm sure you are aware, there is distinction between a shared attribute (inheritance) and two attributes sharing a name (coincidence) e.g. patient may have a legal name attribute and a medication may have a legal name attribute but they do have a shared superclass. why not combine the Contacts and LegalPersons tables? It's probably a 1:m relationship e.g. an organization may have more than one contact. Also, an organization may have a contact that is an individual who has a collection of contacts in the context of being a contact for that organization as well as contacts in their own right e.g. work, home (and they may have more than one home), etc. There is certainly potential for the contacts to be a structu each contact type - postal, email, web, telephone, fax, etc - should have probably be in a table of their own, with perhaps shared super classes (e.g. telephone and fax)... this can quickly get out of hand! One issue here is whether all contact types share a superclass; if so, what would the identifier be? There is a similar issue with the proposed LegalPerson superclass: is there a real life identifier shared by organizations and individuals? Jamie. -- |
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How to enforce subtypes/supertypes in Access 2000?
Hi Graham,
This doesn't seem to be working properly for me. Here's what I've done so far: In tblContacts: - left the pk as it was (ie tblContactsID remains an autonumber pk field) - created a new unique multifield index based on ContactID and ContactTypeID as per the instructions in the Access help file In tblIndividuals: - deleted the original autonumber pk field (IndividualsID) - converted the existing ContactID (number) field as the new pk - inserted a new ContactTypeID (number) field - set the "default value" property to 1 (corresponding to the ContactTypeID for the "Indiv" ContactType in tblContactTypes) - set the "required" property to Yes - created a new unique multifield index based on ContactID and ContactTypeID as per the instructions in the Access help file In tblOrganisations - deleted the original autonumber pk field (OrganisationsID) - converted the existing ContactID (number) field as the new pk - set the "default value" property to 2 (corresponding to the ContactTypeID for the "Orgs" ContactType in tblContactTypes) - set the "required" property to Yes - created a new unique multifield index based on ContactID and ContactTypeID as per the instructions in the Access help file I then created a 1:1 relationship based on the combined ContactID and ContactTypeID fields between tblContacts and tblIndividuals. I did this by selected the two fields in tblContacts and dragging them over to tblIndividuals. I created a 1:1 relationship between tblContacts and tblOrganisations in the same way. I then deleted all existing test data from tblContacts, tblIndividuals and tblOrganisations - starting with a clean slate. I then opened my client data-entry form. The main form has all fields from tblContacts. The subform is unbound, but I have inserted vba into the AfterUpdate event section of the form to ensure that the SourceObject of the subform control is changed to the required subform depending on the selected ContactType (selected from a combo box). I can enter the first record of either tblIndividuals or tblOrganisations without difficulty - once I open the required subform the value of ContactID is the same as the autonumber pk in tblContacts, and the value of the ContactTypeID defaults to the relevant default value. However, after having entered this first record, if I then close and reopen the main form, I am prevented from creating any new reords in either table by a popup box which states "The record cannot be deleted or changed because the table "Individuals" includes related records." What could be causing this error? There are no "related" records in existence! TIA Bob "Graham Mandeno" wrote in message ... Hi Bob The solution that Jamie gave is simple and elegant. To translate it into Access table design terms (which might be more familiar to you than ANSI-92 DDL statements!): Add a unique index to tblContacts involving ContactID AND ContactType. Next add a ContactType field to both tblIndividuals and tblOrganisations and for each, set the default value to the corresponding contact type and set the validation rule to =contact type and set required=Yes. (In other words, an individual MUST be an individual and cannot be an organisation, and vice-versa). Now, add a 1:1 relationship with referential integrity between ContactID/ContactType in tblContacts and tblIndividuals, and the same for tblOrganisations. Now the engine will look after the integrity for you. If a record in tblContacts has a matching record in one of the other tables, then the contact record can neither be deleted, nor changed to the other contact type, unless the related subclass record is first deleted. To answer your other questions: 1. A table-level validation rule can be created in the Table Properties window (ViewProperties in design view). For example: ([IndivID] Is Not Null) Xor ([OrgID] Is Not Null) However, I would NOT use this two-field approach for your particular problem. 2. You can force a prompt before saving a record using the form's BeforeUpdate event. Select case MsgBox("Save changes?", vbYesNoCancel) case vbYes ' do nothing case vbNo Cancel = True Me.Undo case vbCancel cancel = true End Select 3. You can set ANSI-92 mode via ToolsOptionsTables/Queries. Use with caution! -- Good Luck! Graham Mandeno [Access MVP] Auckland, New Zealand "Bob" wrote in message oups.com... Hi Graham, Thanks for the tips. I'll re-examine the use of the autonumbers in the two sub-tables. As to the use of table-level constraints, how do I set this up? I mean, I know I can specify that a particular field can't be null. But how do I force a check on the null value of another field (field B) before allowing one field (field A) to be null? Along the same lines, how can I make sure that at least one field must be have a value? I assume that this would ordinarily be achievable at the form level - but you mention table-level constraints. I'm all ears :-) Going off topic a bit, where the form is concerned I've noticed that my database saves data automatically even if I close the form within pressing save on the toolbar. I assume there is some kind of auto-save when you enter data into a form - but this does not always happen. Sometimes it saves, sometimes it doesn't. How do I force a prompt to save every time the form closes? (I located some example code - which I don't have handy - but it does not seem to work). TIA Bob Graham Mandeno wrote: Hi Bob PMFJI :-) In a Jet (Access) database, there is no way to enforce this sort of entity subclassing at the engine level. The only way to do that would be to have two FK fields in tblContacts - one for IndivID and one for OrgID, and have a table-level constraint (validation rule) to specify that they cannot both be Null. Using the structure you have, you can go most of the way there using a BeforeUpdate event procedure on your ContactType control. Something like this (pseudo-code): If ContactType.OldValue isn't null then lookup corresponding record in table corresponding to OldValue If record exists then Heavy warning message about changing contact type If user wishes to continue then delete old related record else cancel = True End If End If End If BTW, I think you are complicating matters by having separate (AutoNumber?) PKs in your Individuals and Organisations tables. I suggest you make ContactID the PK in both those tables. -- Good Luck! Graham Mandeno [Access MVP] Auckland, New Zealand "Bob" wrote in message ... Thanks for your interest Tina, My full table structure is as follows: tblContacts: ContactID (pk) ContactType (fk) Address1 Address2 City State PostCode PostalAddress1 PostalAddress2 PostalCity PostalState PostalPostCode Tel Fax Mob tblIndividuals: IndivID (pk) ContactID (fk) Title FirstName MiddleNames LastName Suffix EmployerID (fk) (links back to tblContacts.ContactID (1:Many)) EmpDirectPhn EmpDirectFax EmpEmail tblOrganisations: OrgID (pk) ContactID (fk) OrgName TradingName IsACompany (yes/no) ACN (Australian Company Number) ABN (Australian Business Number) Website tblContactType ContactTypeID (pk) ContactType ("Indiv" or "Org") As you can see, tblContacts lists the location (ie residential or business) addresses, postal addresses and (residential or business) telecommunication details for all contacts. The table tblIndividuals segregates the individual-specific biographical information together with the individual's work details. The EmployerID links back to the ContactID field in tblContacts because we often end up acting for employees of existing corporate clients or for muliple employees of non-client organisations. I segregate the Organisation details so that I can record details for all businesses (incorporated and unincorporated (ie sole-proprietorships, partnerships, associations, churches etc)) that simply aren't relevant to individuals. It also enables me to set up a separate table (tblOrgContacts) to identify individual contacts for the organisation entities (a 1:Many relationship is established between the two tables based on tblOrganisations.OrgID (pk) and tblOrgContacts.ContactID (fk)). For our purposes, we do not require any contacts to be linked with Individuals as opposed to Organisations. The above tables essentially constitute the whole set of "contacts" for my employer's business; tblContacts is then linked with tblClients which identifies those contacts that are in fact clients: tlbClients: ClientID (pk) (autonumber) ContactID (fk) (related to tblContacts.ContactID) (1:1 relationship) ReferrerID (fk) (related to tblContacts.ContactID) (1:Many relationship) Regards Bob "tina" wrote in message ... my first thought is: do you really need to to separate the individuals records and organizations records into different tables? suggest you post all the fields in each of those two tables so we can review them; perhaps we can help you combine the two tables into one, with the addition of a single field specifying either "individual" or "organization". hth "Bob" wrote in message ... Hi folks, I am creating a client database in MS Access with the following (simplified) table structu tblContacts: ContactID (pk - autonumber) ContactType (fk) (from tblContactTypes) ContactDetails (text) tblIndividuals IndivID (pk - autonumber) ContactID (fk) (from tblContacts) IndivDetails (txt) tblOrganisations OrgID (pk - autonumber) ContactID (fk) (from tblContacts) OrgDetails (txt) tblContactTypes (serves as a lookup table) tblContactTypeID (pk - autonumber) tblContactType (txt - contains values "Indiv" or "Org") There is a 1:1 relationship between the ContactID (pk) in tblContacts and the ContactID (fks) in tblIndividuals and tbleOrganisations. I have created a form in MS Access for entering client details. At the moment, I have two subforms - frmIndiv and frmOrg - which are positioned on my main entry form. The form contains a combo-box from which the user can select "Indiv" or "Org" as the ContactType. Depending on the value in the combo-box, one or other of the two subforms will become visible. At the moment, the user selects - say - "Indiv" as the ContactType and proceeds to enter details for this type of Contact. When this happens, the ContactID for the current record in tblContacts table is mirrored in the ContactID foreign key in the tblIndividuals table. This is what I want. The problem is that once the user is finished (and whilst still in the same record in the tblContacts table), the user can select "Org" from the combo-box and be provided with a empty copy of the sub-form frmOrg. If the user proceeds to enter data on the sub-form, the ContactID foreign key in the frmOrg will also mirror the ContactID in tblContacts. This results in a record in both of my subtype tables (tblIndividuals and tblOrganisations) having a record which points to the same ContactID in the supertype table (tblContacts). How can I prevent this from happening? - ie make sure that each record in the subtype tables points to a record in the supertype table for which no subtype record has already been created? (That's a mouthful - I hope it makes sense). I've seen some references to "check constraints" on the internet which I believe might help achieve my objective. But - so far as I am aware - I can't impose check constraints on fields in Access 2000. (I have seen a suggestion that this might be achieved by using ADO, but no code example was given). Any pointers would be appreciated. Please note, I am a complete novice at this. TIA Bob |
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