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Which operator is used ...
(match the appropriate)
to check if a subquery returns no rows?
to compare for equality a single value with the result of a subquery?
to compare by more/less a value with the result of a subquery that returns a single column and a single row?
Which type of subquery is executed once for the entire outer query?
Nested subquery
Non-correlated subquery
Scalar subquery
Correlated subquery