I am attempting to work with the SQLEXEC() command and test a SQL table for an empty or Null value in a Date-Time field

I have tried
SQLEXEC(1,'SELECT bbjthead where closedate=NULL','test')
SQLEXEC(1,'SELECT bbjthead where ISNULL(closedate)','test')
SQLEXEC(1,'SELECT bbjthead where closedate={}','test')
SQLEXEC(1,'SELECT bbjthead where closedate=.NULL.','test')

all return -1

However this works so I know my connection opening the table is correct
SQLEXEC(1,'SELECT bbjthead where ljob >"12000"','test')

I realize I must test for Null and {} in SQL Tables where only
EMPTY()was needed in 2.6 FoxPro Tables

Any suggestions?
Thanks