This is an SQL statement and is executed directly on the database.
'SELECT * FROM INVS_DATA
WHERE INVS1_CO_SITE = '01'
AND invscs_order_no IS NOT NULL
AND INVS1_REC_TYPE = 2;'
Can you check the behaviour of the statement on your database using a SQL query tool?
Or, if you want to use the resident load after your initial SQL load, try Len( Trim( Field ) ) > 0.