A scalar subquery expression is a subquery that returns exactly one column value from one row. The value of the scalar subquery expression is the value of the select list item of the subquery. If the subquery returns 0 rows, then the value of the scalar subquery expression is NULL. If the subquery returns more than one row, then Oracle returns an error.
You can use a scalar subquery expression in most syntax that calls for an expression (expr). In all cases, a scalar subquery must be enclosed in its own parentheses, even if its syntactic location already positions it within parentheses (for example, when the scalar subquery is used as the argument to a built-in function).
Scalar subqueries are not valid expressions in the following places:
As default values for columns
As hash expressions for clusters
In the RETURNING clause of DML statements
As the basis of a function-based index
In CHECK constraints
In GROUP BY clauses
In statements that are unrelated to queries, such as CREATE PROFILE