When you use a correlated subquery in a DELETE statement, the correlation name represents the row you delete. SQL evaluates the correlated subquery once for each row in the table named in the DELETE statement to decide whether to delete the row.
Suppose a row in the CORPDATA.PROJECT table was deleted. Rows related to the deleted project in the CORPDATA.EMPPROJACT table must also be deleted. To do this, you can use:
DELETE FROM CORPDATA.EMPPROJACT X
WHERE NOT EXISTS
(SELECT *
FROM CORPDATA.PROJECT
WHERE PROJNO = X.PROJNO)
SQL determines, for each row in the CORPDATA.EMP_ACT table, whether a row with the same project number exists in the CORPDATA.PROJECT table. If not, the CORPDATA.EMP_ACT row is deleted.