Question

I was reading (and am still learning) about the difference between the EXISTS and IN operators in SQL. Is it possible to change the following statement so it uses an EXISTS rather than IN? I’ve tried but the query is taking a very long time to complete. So I’m obviously doing something wrong.

In the below example would it be more efficient to use EXISTS instead of IN? There would be no more than 10 OperationRecordID records returned from the current sub-query used for the IN statement.

SELECT SPAS.SessionRecordID,
       ( SELECT MIN(TimeValue) AS MinAnaes
           FROM tblTMActualTimes TMAT
          WHERE OperationRecordID IN
                 ( SELECT OperationRecordID
                     FROM tblTMActualOperation TMAO
                    WHERE TMAO.SessionRecordID = SPAS.SessionRecordID
                 )
            AND CFTimeDefinitionID = 'INTOANS'
       ) AS FirstAnaesthetic
  FROM tblSPActualSession SPAS

Answer 1

As said in the comments it depends on the data what is more suitable for the situation. But if you what to do it in an exist you can do it something like this:

SELECT 
    SPAS.SessionRecordID,
    ( 
        SELECT 
            MIN(TimeValue) AS MinAnaes
        FROM tblTMActualTimes TMAT
        WHERE EXISTS
                 ( 
                    SELECT 
                        NULL
                    FROM 
                        tblTMActualOperation TMAO
                    WHERE 
                        TMAO.SessionRecordID = SPAS.SessionRecordID
                        AND TMAT.OperationRecordID=TMAO.OperationRecordID
                 )
            AND CFTimeDefinitionID = 'INTOANS'
       ) AS FirstAnaesthetic
  FROM 
    tblSPActualSession SPAS