SQL MIN in Sub query causes huge delay

Posted by Spencer on Stack Overflow See other posts from Stack Overflow or by Spencer
Published on 2010-04-20T14:28:45Z Indexed on 2010/04/20 14:43 UTC
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subquery

I have a SQL query that I'm trying to debug. It works fine for small sets of data, but in large sets of data, this particular part of it causes it to take 45-50 seconds instead of being sub second in speed. This subquery is one of the select items in a larger query. I'm basically trying to figure out when the earliest work date is that fits in the same category as the current row we are looking at (from table dr)

ISNULL(CONVERT(varchar(25),(SELECT MIN(drsd.DateWorked) FROM [TableName] drsd
  WHERE drsd.UserID = dr.UserID
        AND drsd.Val1 = dr.Val1
        OR (((drsd.Val2 = dr.Val2 AND LEN(dr.Val2) > 0) AND (drsd.Val3 = dr.Val3 AND LEN(dr.Val3) > 0) AND (drsd.Val4 = dr.Val4 AND LEN(dr.Val4) > 0))
        OR (drsd.Val5 = dr.Val5 AND LEN(dr.Val5) > 0)
        OR ((drsd.Val6 = dr.Val6 AND LEN(dr.Val6) > 0) AND (drsd.Val7 = dr.Val7 AND LEN(dr.Val2) > 0))))), '') AS WorkStartDate,

This winds up executing a key lookup some 18 million times on a table that has 346,000 records. I've tried creating an index on it, but haven't had any success. Also, selecting a max value in this same query is sub second in time, as it doesn't have to execute very many times at all.

Any suggestions of a different approach to try? Thanks!

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