A scalar subquery expression is a subquery that returns a row. This row contains multiple column values. If a subquery returns zero rows, the value of the scalar subquery expression is NULL. If a subquery returns multiple rows, the scalar query expression returns an error. In most expressions, scalar subquery expressions can be used as parameters.
Scalar subqueries are not valid expressions in the following scenarios:
As default values of columns
In hash functions
In the
RETURNINGclauses of DML statementsIn the definition of a function index
In
CHECKconstraintsIn
GROUP BYclausesIn the statements that are irrelevant to queries, such as
CREATE PROFILE