Ipot: SDE EPY CMA are fixed lenght 3 chars, otherwise you must use the "," separator

SELECT employees.* FROM employees  ;
LEFT JOIN schtbld ON schtbld.schwk = CTOD("04/18/04") and AT(employees.SchedId,schtbld.schres)>0 ;
WHERE schtbld.schres IS NULL

You must have a Index for schtbld.schwk, otherwise split it into two step select, or
use a Locate or LOOKUP

SELECT schtbld
locate for schwk = CTOD("04/18/04")
SELECT * FROM employees  WHERE AT(employees.SchedId,schtbld.schres)=0

or
PRIVATE schwk
schwk = LOOKUP(schtbld.schres,CTOD("04/18/04"),schtbld.schwk)

SELECT * FROM employees  WHERE AT(employees.SchedId,m.schres)=0