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POmysql - UPDATEing row based on other rows
 

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    <p>I'm looking to update certain rows based on specific criteria based on other rows. Lets say the table looks like this:</p> <pre><code>COLUMNS: time type genre doubles triples ROW 1: 2010.06.21 12:00 1 1 0 0 ROW 2: 2010.06.21 12:00 1 2 0 0 ROW 3: 2010.06.21 12:00 1 1 0 0 ROW 4: 2010.06.21 12:00 2 3 0 0 ROW 5: 2010.06.22 12:00 2 2 0 0 ROW 6: 2010.06.22 12:00 2 3 0 0 ROW 7: 2010.06.22 12:00 1 1 0 0 </code></pre> <p>I'm looking to update the <code>doubles</code> and <code>triples</code> columns based on the following rules: </p> <p>1) Only look to update rows where the <code>time=time</code> and <code>type=type</code> between rows (e.g. Rows 1,2,3 and rows 5 &amp; 6). </p> <p>2) Next count the # of different <code>genre</code>'s between those rows and if there are two different <code>genres</code> then change the <code>doubles</code> column to 1, or if there are three then change the <code>triples</code> column to 1. SO for example in the table above rows 1,2,3 would have <code>doubles=1</code> because between the three rows there are two different genres. Rows 5 and 6 would also have <code>doubles=1</code> because there is again two different genres between the rows. <code>Doubles</code> can =1 and <code>triples</code> can =1 but not both.</p> <p>Now, I could write up some PHP based on these rules pretty easily I think, but I'm wondering if there is a way to do it all in mysql? It seems like I'm always surprised the amount you can do from a SQL statement.</p> <p>Maybe something like (two different statements for the <code>doubles</code> and <code>triples</code> column):</p> <p>Doubles - <code>UPDATE myTable SET (doubles=1) WHERE time=time AND type=type</code> ... but then how would you account for rule #2 above (counting number of rows with unique genres).</p> <p>Is this possible in mysql or is PHP just the right way to go here?</p> <p>Thanks in advance</p>
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