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#11
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General solution for missing sequence numbers
As soon as I posted that I realized that there are situations where
sequential numbers are needed - like a hotel where you're dealing with room numbers. (That's a particular OUCH! since I worked in resort operations for 9 years.) Peter Danes wrote: Such situations are common, for a variety of reasons. Depends on the database and the user and what the data is for. The particular example that inspired this outburst is a mycological database, where the numbers are used to sequentially number the scientist's samples. She told me that numbering is important for others in the field to know roughly how many samples a particular researcher has, and for internal inventory purposes, that they don't expect to have holes in the numbering sequence. If someone who has 1,000 samples in their collection publishes something about their sample number 10,000 and it is known that the person does not have anywhere near 10,000 samples, it would be viewed as odd at the very least, possibly unethical and such a person would find himself not taken seriously by other researchers. One or two numbers amiss in this situation is obviously not a major concern. And for the internal inventory controls, if someone sees sample 152 next to 150, they are going to wonder where is number 151. The inventory methods used expect sequential numbering and a missing number is an indication of something wrong. For inventory numbers in the original database, she used the record number that appears in the text box of Access's navigation control in conjunction with an autonumber field. You may guess what sort of hash resulted from that. I started out trying to fix a few things for her and wound up doing almost a complete re-write of the entire thing and this numbering issue is one of the things that surfaced. She wants to be able to fill in all the gaps as well as add new numbers to the end as she collects new samples. Pete "David C. Holley" píse v diskusním príspevku ... What is the specific *NEED* to find the missing numbers? Peter Danes wrote: I occasionally need to determine a number that I don't have in a sequence, either the first missing one in a gap in a set of sequential numbers or the next one in line at the end of a numbered series. Always it meant some fumbling around, with either VBA at first or later with SQL when I got good enough at it, establishing the proper join parameters and such. For SQL experts, this is probably routine and trivial, but for me it was always a bit of a chore. The last straw came with a database which I recently wrote, where the converted data had such a numbered series, and the owner wanted to be able to do both, fill in missing numbers in the gaps AND add new numbers at the end. Walking home from a bar last night, I got to thinking about it and realized that both problems are actually fairly similar and that a simple and general solution is possible. I put together a simple table containing one field with the following entries: 1,2,3,4, 8,9,10, 15,16,17,18, 20, 22,23,24,25, 28,29,30 Missing a 5,6,7, 11,12,13,14, 19, 21, 26,27 and 31 on up. This is the dataset used for all of the following examples. Finding the next new number at the end of a series with SQL is trivial; here is a simplified version of a statement that I found somewhere in the discussion groups a few years ago: SELECT Max(MyTable.MySeqFld)+1 FROM MyTable; This will return a one-record, one-field recordset containing exactly one value: 31, which is one greater than the largest value so far used in that field. This is what you would want to use instead of Access's autonumber, if the field is to contain meaningful sequence numbering, rather than just a unique identifier. Locating gaps is a little more complicated: it involves a self-join from N to N+1 and finding where N+1 doesn't exist, indicating a gap at that point. SELECT MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL; This generates a recordset of 5, 11, 19, 21, 26, 31, where each value is the first missing value in a gap, including the "open gap" at the end, and that's where the trick to a general solution begins. Since these situations normally call for either the first (lowest number) gap or last (end of recordset) gap, you need either the first or last record returned by this query. Sorting and using the TOP predicate gives you exactly that. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld; This will again return a one-record, one-field recordset containing exactly one value: 5, the first missing number in the first gap in the sequence. Ascending sort order is the default, so the smallest number is the first returned. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld DESC; This will return a one-record, one-field recordset containing exactly one value: 31, the same "one greater than the highest value so far used in that field" that is returned by the first simple example. Specifying the descending order here is necessary, since we want the last (greatest) record from the set and Access SQL does not have a BOTTOM predicate. Finally, an even more general statement can be used: SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY ((INSTR("LF",[First or Last (F or L)]) *2)-3)*MT1.MySeqFld; This expects one parameter, F or L and will return either the first missing number or the next number at the end of the line. The INSTR expression evaluates to either -1 or 1 (or -3 if the parameter supplied is neither F nor L, but that has the same effect as -1 in this instance), that is then used as a multiplier for the sort field, so the sort is either by the field or by the negative of the field (or 3 times the negative of the field), giving either ascending or descending order and with the TOP 1 predicate again returns exactly the one value of interest. |
#12
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General solution for missing sequence numbers
Finally, someone who read the sample data! g
-- Doug Steele, Microsoft Access MVP http://I.Am/DougSteele (no e-mails, please!) "Peter Danes" wrote in message ... Small joke. (Very small) In the sample database for the article to which you referred me, you have two tables, PossibleInvoices and Invoices, one with just numbers and one with numbers and a text field. In the text field next to invoice number 11, you had this comment "If so, how about sending me an e-mail?" Sorry for the confusion. Pete "Douglas J Steele" píše v diskusním příspěvku ... I'm sorry, I don't understand what you mean by "invoice 11" -- Doug Steele, Microsoft Access MVP http://I.Am/DougSteele (no e-mails, please!) "Peter Danes" wrote in message ... Thank you Doug, interesting article. I like your addition of the range, I think I'll be able to use that in something I'm working on now. And many of the other titles look intriguing as well - time to do some reading. (BTW, the description for invoice 11 says how about sending me an e-mail, but your signature says no e-mails, please. I'm feeling schizophrenic. Maybe if I write you one but don't send it...?) Pete "Douglas J Steele" píše v diskusním příspěvku ... You might be interested in the analysis I had in my April, 2004 "Access Answers" column in Pinnacle Publication's "Smart Access". You can download the column (and sample database) for free from http://www.accessmvp.com/djsteele/SmartAccess.html -- Doug Steele, Microsoft Access MVP http://I.Am/DougSteele (no e-mails, please!) "Peter Danes" wrote in message ... I occasionally need to determine a number that I don't have in a sequence, either the first missing one in a gap in a set of sequential numbers or the next one in line at the end of a numbered series. Always it meant some fumbling around, with either VBA at first or later with SQL when I got good enough at it, establishing the proper join parameters and such. For SQL experts, this is probably routine and trivial, but for me it was always a bit of a chore. The last straw came with a database which I recently wrote, where the converted data had such a numbered series, and the owner wanted to be able to do both, fill in missing numbers in the gaps AND add new numbers at the end. Walking home from a bar last night, I got to thinking about it and realized that both problems are actually fairly similar and that a simple and general solution is possible. I put together a simple table containing one field with the following entries: 1,2,3,4, 8,9,10, 15,16,17,18, 20, 22,23,24,25, 28,29,30 Missing a 5,6,7, 11,12,13,14, 19, 21, 26,27 and 31 on up. This is the dataset used for all of the following examples. Finding the next new number at the end of a series with SQL is trivial; here is a simplified version of a statement that I found somewhere in the discussion groups a few years ago: SELECT Max(MyTable.MySeqFld)+1 FROM MyTable; This will return a one-record, one-field recordset containing exactly one value: 31, which is one greater than the largest value so far used in that field. This is what you would want to use instead of Access's autonumber, if the field is to contain meaningful sequence numbering, rather than just a unique identifier. Locating gaps is a little more complicated: it involves a self-join from N to N+1 and finding where N+1 doesn't exist, indicating a gap at that point. SELECT MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL; This generates a recordset of 5, 11, 19, 21, 26, 31, where each value is the first missing value in a gap, including the "open gap" at the end, and that's where the trick to a general solution begins. Since these situations normally call for either the first (lowest number) gap or last (end of recordset) gap, you need either the first or last record returned by this query. Sorting and using the TOP predicate gives you exactly that. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld; This will again return a one-record, one-field recordset containing exactly one value: 5, the first missing number in the first gap in the sequence. Ascending sort order is the default, so the smallest number is the first returned. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld DESC; This will return a one-record, one-field recordset containing exactly one value: 31, the same "one greater than the highest value so far used in that field" that is returned by the first simple example. Specifying the descending order here is necessary, since we want the last (greatest) record from the set and Access SQL does not have a BOTTOM predicate. Finally, an even more general statement can be used: SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY ((INSTR("LF",[First or Last (F or L)]) *2)-3)*MT1.MySeqFld; This expects one parameter, F or L and will return either the first missing number or the next number at the end of the line. The INSTR expression evaluates to either -1 or 1 (or -3 if the parameter supplied is neither F nor L, but that has the same effect as -1 in this instance), that is then used as a multiplier for the sort field, so the sort is either by the field or by the negative of the field (or 3 times the negative of the field), giving either ascending or descending order and with the TOP 1 predicate again returns exactly the one value of interest. -- Pete This e-mail address is fake to keep spammers and their auto-harvesters out of my hair. If you need to get in touch personally, I am 'pdanes' and I use Yahoo mail. But please use the newsgroups whenever possible, so that all may benefit from the exchange of ideas. |
#13
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General solution for missing sequence numbers (OT)
Keep your day job.
(FYI - You probably don't want to see some of the remarks that I've left in my code) Peter Danes wrote: Small joke. (Very small) In the sample database for the article to which you referred me, you have two tables, PossibleInvoices and Invoices, one with just numbers and one with numbers and a text field. In the text field next to invoice number 11, you had this comment "If so, how about sending me an e-mail?" Sorry for the confusion. Pete "Douglas J Steele" píše v diskusním příspěvku ... I'm sorry, I don't understand what you mean by "invoice 11" -- Doug Steele, Microsoft Access MVP http://I.Am/DougSteele (no e-mails, please!) "Peter Danes" wrote in message .. . Thank you Doug, interesting article. I like your addition of the range, I think I'll be able to use that in something I'm working on now. And many of the other titles look intriguing as well - time to do some reading. (BTW, the description for invoice 11 says how about sending me an e-mail, but your signature says no e-mails, please. I'm feeling schizophrenic. Maybe if I write you one but don't send it...?) Pete "Douglas J Steele" píše v diskusním příspěvku ... You might be interested in the analysis I had in my April, 2004 "Access Answers" column in Pinnacle Publication's "Smart Access". You can download the column (and sample database) for free from http://www.accessmvp.com/djsteele/SmartAccess.html -- Doug Steele, Microsoft Access MVP http://I.Am/DougSteele (no e-mails, please!) "Peter Danes" wrote in message ... I occasionally need to determine a number that I don't have in a sequence, either the first missing one in a gap in a set of sequential numbers or the next one in line at the end of a numbered series. Always it meant some fumbling around, with either VBA at first or later with SQL when I got good enough at it, establishing the proper join parameters and such. For SQL experts, this is probably routine and trivial, but for me it was always a bit of a chore. The last straw came with a database which I recently wrote, where the converted data had such a numbered series, and the owner wanted to be able to do both, fill in missing numbers in the gaps AND add new numbers at the end. Walking home from a bar last night, I got to thinking about it and realized that both problems are actually fairly similar and that a simple and general solution is possible. I put together a simple table containing one field with the following entries: 1,2,3,4, 8,9,10, 15,16,17,18, 20, 22,23,24,25, 28,29,30 Missing a 5,6,7, 11,12,13,14, 19, 21, 26,27 and 31 on up. This is the dataset used for all of the following examples. Finding the next new number at the end of a series with SQL is trivial; here is a simplified version of a statement that I found somewhere in the discussion groups a few years ago: SELECT Max(MyTable.MySeqFld)+1 FROM MyTable; This will return a one-record, one-field recordset containing exactly one value: 31, which is one greater than the largest value so far used in that field. This is what you would want to use instead of Access's autonumber, if the field is to contain meaningful sequence numbering, rather than just a unique identifier. Locating gaps is a little more complicated: it involves a self-join from N to N+1 and finding where N+1 doesn't exist, indicating a gap at that point. SELECT MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL; This generates a recordset of 5, 11, 19, 21, 26, 31, where each value is the first missing value in a gap, including the "open gap" at the end, and that's where the trick to a general solution begins. Since these situations normally call for either the first (lowest number) gap or last (end of recordset) gap, you need either the first or last record returned by this query. Sorting and using the TOP predicate gives you exactly that. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld; This will again return a one-record, one-field recordset containing exactly one value: 5, the first missing number in the first gap in the sequence. Ascending sort order is the default, so the smallest number is the first returned. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld DESC; This will return a one-record, one-field recordset containing exactly one value: 31, the same "one greater than the highest value so far used in that field" that is returned by the first simple example. Specifying the descending order here is necessary, since we want the last (greatest) record from the set and Access SQL does not have a BOTTOM predicate. Finally, an even more general statement can be used: SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY ((INSTR("LF",[First or Last (F or L)]) *2)-3)*MT1.MySeqFld; This expects one parameter, F or L and will return either the first missing number or the next number at the end of the line. The INSTR expression evaluates to either -1 or 1 (or -3 if the parameter supplied is neither F nor L, but that has the same effect as -1 in this instance), that is then used as a multiplier for the sort field, so the sort is either by the field or by the negative of the field (or 3 times the negative of the field), giving either ascending or descending order and with the TOP 1 predicate again returns exactly the one value of interest. -- Pete This e-mail address is fake to keep spammers and their auto-harvesters out of my hair. If you need to get in touch personally, I am 'pdanes' and I use Yahoo mail. But please use the newsgroups whenever possible, so that all may benefit from the exchange of ideas. |
#14
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General solution for missing sequence numbers
On Mon, 5 Dec 2005 18:53:45 +0100, "Peter Danes"
wrote: Such situations are common, for a variety of reasons. Depends on the database and the user and what the data is for. The particular example that inspired this outburst is a mycological database, where the numbers are used to sequentially number the scientist's samples. She told me that numbering is important for others in the field to know roughly how many samples a particular researcher has, and for internal inventory purposes, that they don't expect to have holes in the numbering sequence. I agree that Access' autonumber isn't suitable in such cases... but there are real, major problems with "filling in the gaps." How did the gap get there in the first place? Presumably a Sample #312 was entered at some point, and then deleted: the entry was found to be erroneous, misnumbered, or for some other reason had to be removed. New entries would go in up at Sample #844 but you now have this gap. OK... fill in the gap then. BUT! What if there's a publication referring to the (erroneous) old Sample 312, and you now assign a DIFFERENT sample the same number? What if someone has 312 written down on a Post-It note as "check up on this really interesting sample" - or noted in their memory? Sure, you can change it in the database; but where *else* does the information exist, and can you change *that*? John W. Vinson[MVP] |
#15
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General solution for missing sequence numbers
To summarize, I believe that the general question is - What is happening
that is causing the gaps to occurr? John Vinson wrote: On Mon, 5 Dec 2005 18:53:45 +0100, "Peter Danes" wrote: Such situations are common, for a variety of reasons. Depends on the database and the user and what the data is for. The particular example that inspired this outburst is a mycological database, where the numbers are used to sequentially number the scientist's samples. She told me that numbering is important for others in the field to know roughly how many samples a particular researcher has, and for internal inventory purposes, that they don't expect to have holes in the numbering sequence. I agree that Access' autonumber isn't suitable in such cases... but there are real, major problems with "filling in the gaps." How did the gap get there in the first place? Presumably a Sample #312 was entered at some point, and then deleted: the entry was found to be erroneous, misnumbered, or for some other reason had to be removed. New entries would go in up at Sample #844 but you now have this gap. OK... fill in the gap then. BUT! What if there's a publication referring to the (erroneous) old Sample 312, and you now assign a DIFFERENT sample the same number? What if someone has 312 written down on a Post-It note as "check up on this really interesting sample" - or noted in their memory? Sure, you can change it in the database; but where *else* does the information exist, and can you change *that*? John W. Vinson[MVP] |
#16
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General solution for missing sequence numbers
Peter Danes wrote:
I occasionally need to determine a number that I don't have in a sequence, either the first missing one in a gap in a set of sequential numbers or the next one in line at the end of a numbered series. Always it meant some fumbling around, with either VBA at first or later with SQL when I got good enough at it, establishing the proper join parameters and such. For SQL experts, this is probably routine and trivial, but for me it was always a bit of a chore. The last straw came with a database which I recently wrote, where the converted data had such a numbered series, and the owner wanted to be able to do both, fill in missing numbers in the gaps AND add new numbers at the end. Walking home from a bar last night, I got to thinking about it and realized that both problems are actually fairly similar and that a simple and general solution is possible. I put together a simple table containing one field with the following entries: 1,2,3,4, 8,9,10, 15,16,17,18, 20, 22,23,24,25, 28,29,30 Missing a 5,6,7, 11,12,13,14, 19, 21, 26,27 and 31 on up. This is the dataset used for all of the following examples. Finding the next new number at the end of a series with SQL is trivial; here is a simplified version of a statement that I found somewhere in the discussion groups a few years ago: SELECT Max(MyTable.MySeqFld)+1 FROM MyTable; This will return a one-record, one-field recordset containing exactly one value: 31, which is one greater than the largest value so far used in that field. This is what you would want to use instead of Access's autonumber, if the field is to contain meaningful sequence numbering, rather than just a unique identifier. Locating gaps is a little more complicated: it involves a self-join from N to N+1 and finding where N+1 doesn't exist, indicating a gap at that point. SELECT MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL; This generates a recordset of 5, 11, 19, 21, 26, 31, where each value is the first missing value in a gap, including the "open gap" at the end, and that's where the trick to a general solution begins. Since these situations normally call for either the first (lowest number) gap or last (end of recordset) gap, you need either the first or last record returned by this query. Sorting and using the TOP predicate gives you exactly that. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld; This will again return a one-record, one-field recordset containing exactly one value: 5, the first missing number in the first gap in the sequence. Ascending sort order is the default, so the smallest number is the first returned. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld DESC; This will return a one-record, one-field recordset containing exactly one value: 31, the same "one greater than the highest value so far used in that field" that is returned by the first simple example. Specifying the descending order here is necessary, since we want the last (greatest) record from the set and Access SQL does not have a BOTTOM predicate. Finally, an even more general statement can be used: SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY ((INSTR("LF",[First or Last (F or L)]) *2)-3)*MT1.MySeqFld; This expects one parameter, F or L and will return either the first missing number or the next number at the end of the line. The INSTR expression evaluates to either -1 or 1 (or -3 if the parameter supplied is neither F nor L, but that has the same effect as -1 in this instance), that is then used as a multiplier for the sort field, so the sort is either by the field or by the negative of the field (or 3 times the negative of the field), giving either ascending or descending order and with the TOP 1 predicate again returns exactly the one value of interest. An alternative SQL method using subqueries can be found he http://groups.google.com/group/micro...4a5fee9ff13437 James A. Fortune I was staying at a hotel and decided to go down to the pool. A 13 year old bratty kid splashed water on me. When I told him to stop he said, "You can't make me. I'm a minor. If you touch me you'll get in a lot of trouble." I found a 17 year old at the pool and paid him $5 to hold the kid underwater for 10 seconds. "How do you like me now?" -- Rob Smith |
#17
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General solution for missing sequence numbers
In that situation the issue isn't about how to 'fill in the gaps', it's how to explain to a particular group of scientists that they don't have a clue about how relational databases work. Are they really stupid enough to not understand that, although my highest ID number is 10000, that I only have 1000 samples? Even if I point out that 9000 of those records are marked as 'not really a sample' (i.e. the idea of disallowing deletions and marking the record as inactive instead)? Expecting sequential numbering is just wrong. It's up to those of us who actually know about the issues raised by John Vinson to not allow those who don't to tell us how to put databases together. "Peter Danes" wrote in message ... Such situations are common, for a variety of reasons. Depends on the database and the user and what the data is for. The particular example that inspired this outburst is a mycological database, where the numbers are used to sequentially number the scientist's samples. She told me that numbering is important for others in the field to know roughly how many samples a particular researcher has, and for internal inventory purposes, that they don't expect to have holes in the numbering sequence. If someone who has 1,000 samples in their collection publishes something about their sample number 10,000 and it is known that the person does not have anywhere near 10,000 samples, it would be viewed as odd at the very least, possibly unethical and such a person would find himself not taken seriously by other researchers. One or two numbers amiss in this situation is obviously not a major concern. And for the internal inventory controls, if someone sees sample 152 next to 150, they are going to wonder where is number 151. The inventory methods used expect sequential numbering and a missing number is an indication of something wrong. For inventory numbers in the original database, she used the record number that appears in the text box of Access's navigation control in conjunction with an autonumber field. You may guess what sort of hash resulted from that. I started out trying to fix a few things for her and wound up doing almost a complete re-write of the entire thing and this numbering issue is one of the things that surfaced. She wants to be able to fill in all the gaps as well as add new numbers to the end as she collects new samples. Pete "David C. Holley" píse v diskusním príspevku ... What is the specific *NEED* to find the missing numbers? Peter Danes wrote: I occasionally need to determine a number that I don't have in a sequence, either the first missing one in a gap in a set of sequential numbers or the next one in line at the end of a numbered series. Always it meant some fumbling around, with either VBA at first or later with SQL when I got good enough at it, establishing the proper join parameters and such. For SQL experts, this is probably routine and trivial, but for me it was always a bit of a chore. The last straw came with a database which I recently wrote, where the converted data had such a numbered series, and the owner wanted to be able to do both, fill in missing numbers in the gaps AND add new numbers at the end. Walking home from a bar last night, I got to thinking about it and realized that both problems are actually fairly similar and that a simple and general solution is possible. I put together a simple table containing one field with the following entries: 1,2,3,4, 8,9,10, 15,16,17,18, 20, 22,23,24,25, 28,29,30 Missing a 5,6,7, 11,12,13,14, 19, 21, 26,27 and 31 on up. This is the dataset used for all of the following examples. Finding the next new number at the end of a series with SQL is trivial; here is a simplified version of a statement that I found somewhere in the discussion groups a few years ago: SELECT Max(MyTable.MySeqFld)+1 FROM MyTable; This will return a one-record, one-field recordset containing exactly one value: 31, which is one greater than the largest value so far used in that field. This is what you would want to use instead of Access's autonumber, if the field is to contain meaningful sequence numbering, rather than just a unique identifier. Locating gaps is a little more complicated: it involves a self-join from N to N+1 and finding where N+1 doesn't exist, indicating a gap at that point. SELECT MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL; This generates a recordset of 5, 11, 19, 21, 26, 31, where each value is the first missing value in a gap, including the "open gap" at the end, and that's where the trick to a general solution begins. Since these situations normally call for either the first (lowest number) gap or last (end of recordset) gap, you need either the first or last record returned by this query. Sorting and using the TOP predicate gives you exactly that. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld; This will again return a one-record, one-field recordset containing exactly one value: 5, the first missing number in the first gap in the sequence. Ascending sort order is the default, so the smallest number is the first returned. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld DESC; This will return a one-record, one-field recordset containing exactly one value: 31, the same "one greater than the highest value so far used in that field" that is returned by the first simple example. Specifying the descending order here is necessary, since we want the last (greatest) record from the set and Access SQL does not have a BOTTOM predicate. Finally, an even more general statement can be used: SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY ((INSTR("LF",[First or Last (F or L)]) *2)-3)*MT1.MySeqFld; This expects one parameter, F or L and will return either the first missing number or the next number at the end of the line. The INSTR expression evaluates to either -1 or 1 (or -3 if the parameter supplied is neither F nor L, but that has the same effect as -1 in this instance), that is then used as a multiplier for the sort field, so the sort is either by the field or by the negative of the field (or 3 times the negative of the field), giving either ascending or descending order and with the TOP 1 predicate again returns exactly the one value of interest. |
#18
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General solution for missing sequence numbers
So why not simply create the sample numbers by hand and then enter them?
I would hope that any scientist capable of research would also be capable of counting from 1 to 1000. Rob Oldfield wrote: In that situation the issue isn't about how to 'fill in the gaps', it's how to explain to a particular group of scientists that they don't have a clue about how relational databases work. Are they really stupid enough to not understand that, although my highest ID number is 10000, that I only have 1000 samples? Even if I point out that 9000 of those records are marked as 'not really a sample' (i.e. the idea of disallowing deletions and marking the record as inactive instead)? Expecting sequential numbering is just wrong. It's up to those of us who actually know about the issues raised by John Vinson to not allow those who don't to tell us how to put databases together. "Peter Danes" wrote in message ... Such situations are common, for a variety of reasons. Depends on the database and the user and what the data is for. The particular example that inspired this outburst is a mycological database, where the numbers are used to sequentially number the scientist's samples. She told me that numbering is important for others in the field to know roughly how many samples a particular researcher has, and for internal inventory purposes, that they don't expect to have holes in the numbering sequence. If someone who has 1,000 samples in their collection publishes something about their sample number 10,000 and it is known that the person does not have anywhere near 10,000 samples, it would be viewed as odd at the very least, possibly unethical and such a person would find himself not taken seriously by other researchers. One or two numbers amiss in this situation is obviously not a major concern. And for the internal inventory controls, if someone sees sample 152 next to 150, they are going to wonder where is number 151. The inventory methods used expect sequential numbering and a missing number is an indication of something wrong. For inventory numbers in the original database, she used the record number that appears in the text box of Access's navigation control in conjunction with an autonumber field. You may guess what sort of hash resulted from that. I started out trying to fix a few things for her and wound up doing almost a complete re-write of the entire thing and this numbering issue is one of the things that surfaced. She wants to be able to fill in all the gaps as well as add new numbers to the end as she collects new samples. Pete "David C. Holley" píse v diskusním príspevku .. . What is the specific *NEED* to find the missing numbers? Peter Danes wrote: I occasionally need to determine a number that I don't have in a sequence, either the first missing one in a gap in a set of sequential numbers or the next one in line at the end of a numbered series. Always it meant some fumbling around, with either VBA at first or later with SQL when I got good enough at it, establishing the proper join parameters and such. For SQL experts, this is probably routine and trivial, but for me it was always a bit of a chore. The last straw came with a database which I recently wrote, where the converted data had such a numbered series, and the owner wanted to be able to do both, fill in missing numbers in the gaps AND add new numbers at the end. Walking home from a bar last night, I got to thinking about it and realized that both problems are actually fairly similar and that a simple and general solution is possible. I put together a simple table containing one field with the following entries: 1,2,3,4, 8,9,10, 15,16,17,18, 20, 22,23,24,25, 28,29,30 Missing a 5,6,7, 11,12,13,14, 19, 21, 26,27 and 31 on up. This is the dataset used for all of the following examples. Finding the next new number at the end of a series with SQL is trivial; here is a simplified version of a statement that I found somewhere in the discussion groups a few years ago: SELECT Max(MyTable.MySeqFld)+1 FROM MyTable; This will return a one-record, one-field recordset containing exactly one value: 31, which is one greater than the largest value so far used in that field. This is what you would want to use instead of Access's autonumber, if the field is to contain meaningful sequence numbering, rather than just a unique identifier. Locating gaps is a little more complicated: it involves a self-join from N to N+1 and finding where N+1 doesn't exist, indicating a gap at that point. SELECT MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL; This generates a recordset of 5, 11, 19, 21, 26, 31, where each value is the first missing value in a gap, including the "open gap" at the end, and that's where the trick to a general solution begins. Since these situations normally call for either the first (lowest number) gap or last (end of recordset) gap, you need either the first or last record returned by this query. Sorting and using the TOP predicate gives you exactly that. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld; This will again return a one-record, one-field recordset containing exactly one value: 5, the first missing number in the first gap in the sequence. Ascending sort order is the default, so the smallest number is the first returned. SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY MT1.MySeqFld DESC; This will return a one-record, one-field recordset containing exactly one value: 31, the same "one greater than the highest value so far used in that field" that is returned by the first simple example. Specifying the descending order here is necessary, since we want the last (greatest) record from the set and Access SQL does not have a BOTTOM predicate. Finally, an even more general statement can be used: SELECT TOP 1 MT1.MySeqFld+1 FROM MyTable AS MT1 LEFT JOIN MyTable AS MT2 ON MT1.MySeqFld+1=MT2.MySeqFld WHERE MT2.MySeqFld IS NULL ORDER BY ((INSTR("LF",[First or Last (F or L)]) *2)-3)*MT1.MySeqFld; This expects one parameter, F or L and will return either the first missing number or the next number at the end of the line. The INSTR expression evaluates to either -1 or 1 (or -3 if the parameter supplied is neither F nor L, but that has the same effect as -1 in this instance), that is then used as a multiplier for the sort field, so the sort is either by the field or by the negative of the field (or 3 times the negative of the field), giving either ascending or descending order and with the TOP 1 predicate again returns exactly the one value of interest. |
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General solution for missing sequence numbers
"Peter Danes" wrote in message ... "Chris2" "Peter Danes" wrote in message ... snip snip Sincerely, Chris O. There are three differences: 1. Your example is the same as my first example which returns only the "greatest +1", except that you additionally include an alias to the table, the "AS MT1" at the end of the statement. It doesn't hurt anything, but isn't really necessary. Using table aliases may not be necessary, but I haven't written a query more complicated than SELECT * FROM table_name in years without them. The readability of SQL is greatly improved by their use, and some queries cannot be written without them. In any event, table aliases were not the purpose of my post. I was only asking a question. Sincerely, Chris O. |
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General solution for missing sequence numbers
Hello John.
The gaps got there in the first place because the owner of the database is a scientist, not a computer tech. As I explained in response to David's post, she was using the record number that appears in a form's navigation text box as an identifier, and trying to keep that synchronized with an autonumber field. After discovering that a deleted record, even at the end, did not re-use the next available number (the standard autonumber lament of beginners) she went at it by erasing text from the individual fields and entering new text without actually deleting the record. But by the time she discovered this kludge, she already had gaps in the numbering. I had quite a time convincing her that the record number in the navigation box is not an identifier and that her whole approach to this was not the best way to handle the numbering issue. As far as the problem of references to specific numbers in other places than the database, you're right, but it's not something I can address. This is the way their system works, they like it that way and want it left alone. And in this case, I don't believe it's really a problem. Most of the gaps are places where something was written down incorrectly, inadvertently erased and the old autonumber field wouldn't let them use the number again. Or some numbers were not used at all - there was a conversion somewhere along the line, trying to get the record number back in sync with autonumber and someone managed to sling in a gap of 230 unused numbers. I'm not sure quite how they managed that, when I asked about it and what they had done, the general response was that they didn't know what they had done, it just somehow turned out that way. Again, these are scientists, not computer techs. In any case, this dataset is what I got handed and I had to do what I could to accomodate their needs. A better identifying scheme is certainly not difficult to imagine, but I can't really expect them to drop what they're doing and go renumber their entire collection just to suit my technical preferences. -- Pete "John Vinson" píse v diskusním príspevku ... On Mon, 5 Dec 2005 18:53:45 +0100, "Peter Danes" wrote: Such situations are common, for a variety of reasons. Depends on the database and the user and what the data is for. The particular example that inspired this outburst is a mycological database, where the numbers are used to sequentially number the scientist's samples. She told me that numbering is important for others in the field to know roughly how many samples a particular researcher has, and for internal inventory purposes, that they don't expect to have holes in the numbering sequence. I agree that Access' autonumber isn't suitable in such cases... but there are real, major problems with "filling in the gaps." How did the gap get there in the first place? Presumably a Sample #312 was entered at some point, and then deleted: the entry was found to be erroneous, misnumbered, or for some other reason had to be removed. New entries would go in up at Sample #844 but you now have this gap. OK... fill in the gap then. BUT! What if there's a publication referring to the (erroneous) old Sample 312, and you now assign a DIFFERENT sample the same number? What if someone has 312 written down on a Post-It note as "check up on this really interesting sample" - or noted in their memory? Sure, you can change it in the database; but where *else* does the information exist, and can you change *that*? John W. Vinson[MVP] |
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