Question

I am working in a project which needs top performance in SQL results, and was looking to optimize a query, but after some trial and error I am having some trouble with IN.

-- THIS RETURNS NO RESULTS AT ALL.
SELECT sysdba.HISTORY.TICKETID FROM sysdba.HISTORY  
     where TICKETID = 't6UJ9A002MJC'                                         

-- THIS RETURNS 4 RESULTS.
SELECT sysdba.C_TICKETPROVIDERS.* FROM sysdba.C_TICKETPROVIDERS 
     where sysdba.C_TICKETPROVIDERS.TICKETID = 't6UJ9A002MJC'

-- THIS RETURNS NO RESULTS AT ALL.
SELECT sysdba.C_TICKETPROVIDERS.* FROM sysdba.C_TICKETPROVIDERS 
     where sysdba.C_TICKETPROVIDERS.TICKETID NOT IN 
        (SELECT DISTINCT sysdba.HISTORY.TICKETID FROM sysdba.HISTORY) 

Shouldn’t the last query result at least the same 4 results as the previous one, since there are no matching results???

Answer 1

The piece NOT IN (NULL) in your WHERE clausole returns false.

Try with:

SELECT sysdba.C_TICKETPROVIDERS.* FROM sysdba.C_TICKETPROVIDERS 
     where NOT EXISTS 
         (SELECT TOP 1 1 
          FROM sysdba.HISTORY
          WHERE sysdba.HISTORY.TICKETID  = sysdba.C_TICKETPROVIDERS.TICKETID) 

• The subquery seeks for at least one record.
  • If it finds it, returns 1, and the NOT EXISTS clausole fails, as expected.
  • If it doesn’t find it, returns NULL, and the NOT EXISTS will succeed.