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POUsing TSQL, how can I tell if my CHAR is last one in a sequence? Also, what if the letter sequences "rolls over" from 'z' to 'a'?
 

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  1. POUsing TSQL, how can I tell if my CHAR is last one in a sequence? Also, what if the letter sequences "rolls over" from 'z' to 'a'?
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    <p>Let's say I have the following table in SQL Server 2005:</p> <pre><code>id | Date | Letter ----------------------------- 01 | 5/1/2009 | W 02 | 5/1/2009 | X 03 | 5/1/2009 | Y 04 | 5/1/2009 | Z 05 | 5/1/2009 | A 06 | 5/1/2009 | B 07 | 5/1/2009 | D 08 | 5/1/2009 | E 09 | 5/2/2009 | W 10 | 5/2/2009 | X 11 | 5/2/2009 | Y 12 | 5/2/2009 | Z 13 | 5/2/2009 | A 14 | 5/2/2009 | B 15 | 5/2/2009 | C 16 | 5/2/2009 | D 17 | 5/3/2009 | W 18 | 5/3/2009 | X </code></pre> <p>In my <a href="https://stackoverflow.com/questions/938357/using-tsql-can-i-increment-a-char1-column-by-one-and-use-it-in-a-left-outer-jo/938371#938371">previous question</a>, I asked how to figure out which letter sequence was missing. In my example it would letter C between row 6 and 7. I got the following answer and it works for the most part:</p> <pre><code>SELECT * from TABLE1 t1 LEFT OUTER JOIN TABLE2 t2 ON ASCII(t2.INTCol) - 1 = ASCII(t2.INTCol) AND t1.date = t2.date WHERE t2.id IS NULL </code></pre> <p>However, this solution does have two problems. It will return false positives in two cases.</p> <ol> <li><p>The letter sequence starts over at 'W' everyday. So, I get a false positive result for the last record of every day. That would be row 8 and 16 in my example data.</p></li> <li><p>Since the sequence starts over at 'W' everyday, I get a false positive for the 'Z' record because ('Z' + 1) gives me a '[', not an 'A'.</p></li> </ol> <p>For #2 I believe I can put in an "IF" statement that says: "If this is a Z, look for an A" but I'm open to any 'cleaner' solutions.</p> <p>I really have no idea how to do #1, so I'm really looking for help on that one. <strong>EDIT:</strong> Something I left out, doing a MAX(t1.id) isn't going to work, as the rows aren't always inserted in sequence. In fact, looking at my data, it's always inserted in [a-z] order even though the first record is the 'w'. I know it's lame, but I'm dealing with log files from dozens of million dollar phone switches. Not much I can do about it.</p>
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    1. USNick Messick
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      1. POUsing TSQL, how can I tell if my CHAR is last one in a sequence? Also, what if the letter sequences "rolls over" from 'z' to 'a'?
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    1. COHow should the procedure know what Letter it is supposed to start and end at? How do you know that there should not have been an "F" on 5/1 and that the sequence warning on row 8 is not correct? Likewise, maybe 5/2 really should have started on a "V"? What information is avialable to tell us that?
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    2. COThe sequence always starts with a "W", no matter what. And yeah, it will be impossible to know if the very last Letter(s) is/are missing. That shortcoming is something I'll have to live with.
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      1. USNick Messick
 

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