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Exists & Not Exists

the concepts of "exists" and "not exists" are typically used with subqueries to check for the presence or absence of rows that meet certain criteria. The EXISTS keyword is used to test for the existence of any record in a subquery. It returns TRUE if the subquery returns one or more rows. Conversely, NOT EXISTS checks for the non-existence of records in a subquery. It returns TRUE if the subquery returns no rows. EXISTS is often more efficient than using JOIN for checking existence because it can stop processing once it finds the first matching row.