It is a column that is not an actual column in the table.
eg USER, UID, SYSDATE, ROWNUM, ROWID, NULL, AND LEVEL.
Suppose customer table is there having different columns like customer no, payments.What will be the query to select top three max payments.
SELECT customer_no, payments from customer C1 WHERE 3<=(SELECT COUNT(*) from customer C2 WHERE C1.payment <= C2.payment)
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