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POHow to query previous rows of a table in a select statement if the table is resorted by a non-primary key
 

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  1. POHow to query previous rows of a table in a select statement if the table is resorted by a non-primary key
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    <p>I have a table named Clients with three columns - RowID, ClientNumber, and ClientNote where RowID is the primary key. Each ClientNumber has multiple ClientNotes so that the data looks like this:</p> <pre><code>RowId ClientNumber ClientNote 1 678 Note 1 2 678 Note 2 3 123 Note 3 4 123 Note 4 5 123 Note 1 6 F45 Note 3 7 F45 Note 6 </code></pre> <p>I am trying to return a table that sorts the data by ClientNumber, and also adds a column that could be used to print different color backgrounds if needed:</p> <pre><code>ClientNumber ClientNote Color 123 Note 3 Blue 123 Note 4 Blue 123 Note 1 White 678 Note 1 White 678 Note 2 White F45 Note 3 Blue F45 Note 6 Blue </code></pre> <p>I am trying to use a CASE statement, where my base case (row 1) is set to Blue, and I would like to then access the previous row to query data to make a decision on color.</p> <p>Here is my code:</p> <pre><code>SELECT ClientNumber , ClientNote, CASE WHEN (SELECT @rownum:=@rownum+1 rownum) = 1 THEN 'Blue' WHEN @rownum != 1 AND (SELECT ClientNumber FROM Clients WHERE @rownum - 1 &lt; @rownum ORDER BY ClientNumber ASC LIMIT 1) = ClientNumber AND (SELECT 'Color' FROM Clients WHERE @rownum - 1 &lt; @rownum ORDER BY ClientNumber ASC LIMIT 1) = 'White' THEN 'White' WHEN @rownum != 1 AND (SELECT ClientNumber FROM Clients WHERE @rownum - 1 &lt; @rownum ORDER BY ClientNumber ASC LIMIT 1) = ClientNumber AND (SELECT 'Color' FROM Clients WHERE @rownum - 1 &lt; @rownum ORDER BY ClientNumber ASC LIMIT 1) = 'Blue' THEN 'Blue' WHEN @rownum != 1 AND (SELECT ClientNumber FROM Clients WHERE @rownum - 1 &lt; @rownum ORDER BY ClientNumber ASC LIMIT 1) != ClientNumber AND (SELECT 'Color' FROM Clients WHERE @rownum - 1 &lt; @rownum ORDER BY ClientNumber ASC LIMIT 1) = 'Blue' THEN 'White' WHEN @rownum != 1 AND (SELECT ClientNumber FROM Clients WHERE @rownum - 1 &lt; @rownum ORDER BY ClientNumber ASC LIMIT 1) != ClientNumber AND (SELECT 'Color' FROM Clients WHERE @rownum - 1 &lt; @rownum ORDER BY ClientNumber ASC LIMIT 1) = 'White' THEN 'Blue' END AS Color FROM (SELECT @rownum:=0) r, Clients ORDER BY ClientNumber ASC </code></pre> <p>Essentially, I am just looking back to see if the previous ClientNumber matches or not, and inserting a color based on the previous color. I am having a problem querying the ClientNumber on the previous row in my new table. I am using MySQL-server, and I don't know a lot about SQL. </p> <p>The other problem I am having is querying the color of the previous row. Since I am using a CASE statement, I think I might have to access the column information instead of using the name of the column, but I am not sure if this is necessary. </p> <p>The problem and solution are pretty basic, but the syntax is killing me!! I've tried just about everything I can think of (the above code is the latest version) except joins, which I don't even really understand.</p> <p>Any explanation of how to access "previous" (if they can really be called that in a database) rows and CASE statement results would really be appreciated.</p>
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