The exists predicate is used to test if a subquery returns any rows.
Syntax
<exists predicate> ::= EXISTS '(' <query specification> ')'.
The result of the <exists predicate> is true if the result of the <query specification> is not empty (contains one or more rows), otherwise the result of the <exists predicate> is false.
Syntax
SELECT * FROM a WHERE EXISTS ( SELECT * FROM b WHERE b.c = 0 )
The EXISTS Predicate. This query contains an uncorrelated subquery. It returns all rows from table a if there is at least one row in table b for which the column c of table b has the value 0.