NEED HELP!!!! Oracle equivalent for Sybase SQL...

Here's the code... I'm having problems with the testing of the subquery for nulls. Oracle needs the format 'expr operator subquery' and 'NULL != query' of course won't work. HELP!

  update TOTAL_COMP_DATABASE

        SET LOA_DATE = 


        (SELECT MIN (J3.EFFDT) FROM PS_JOB J3


          WHERE J3.EMPLID = TOTAL_COMP_DATABASE.EMPLID AND J3.ACTION = 'LOA'AND 
          J3.ACTION_REASON != 'PAR' AND J3.EFFDT >= 
          TOTAL_COMP_DATABASE.RET_DATE) 
        WHERE


        (SELECT MIN (J3.EFFDT) FROM PS_JOB J3


          WHERE J3.EMPLID = TOTAL_COMP_DATABASE.EMPLID AND J3.ACTION = 'LOA' AND 
          J3.ACTION_REASON != 'PAR' AND J3.EFFDT >= 
          TOTAL_COMP_DATABASE.RET_DATE) != NULL AND RET_DATE != NULL

	-kibwe_at_wpi.wpi.edu