Subqueries¶

Subqueries are scalar expressions comprised of another query.

Forms¶

Scalar subqueries are subqueries that return one row and one column.

Property	Description	Required
Input	Input relation	Yes

An IN subquery predicate checks that the left expression is contained in the right subquery.

SELECT *
FROM t1
WHERE x IN (SELECT * FROM t2)

SELECT *
FROM t1
WHERE (x, y) IN (SELECT a, b FROM t2)

Property	Description	Required
Needles	Expressions whose existence will be checked	Yes
Haystack	Subquery to check	Yes

A set predicate is a predicate over a set of rows in the form of a subquery.

EXISTS and UNIQUE are common SQL spellings of these kinds of predicates.

Property	Description	Required
Operation	The operation to perform over the set	Yes
Tuples	Set of tuples to check using the operation	Yes

A set comparison subquery is a subquery comparison using ANY or ALL operations.

SELECT *
FROM t1
WHERE x < ANY(SELECT y from t2)

Property	Description	Required
Reduction operation	The kind of reduction to use over the subquery	Yes
Comparison operation	The kind of comparison operation to use	Yes
Expression	Left-hand side expression to check	Yes
Subquery	Subquery to check	Yes