Scalar Subquery
Scalar Subquery is a SQL statement in the Subqueries category. A subquery that returns a single value (one row, one column). Can be used in SELECT, WHERE, HAVING clauses. The syntax is SELECT column, (SELECT aggregate FROM table2) FROM table1;. It returns single value. A typical example: SELECT name, salary, (SELECT AVG(salary) FROM employees) AS company_avg FROM employees; -- Compare each salary to company average A close relative is Correlated Subquery, which a subquery that references columns from the outer query. Evaluated for each row of the outer query. A close relative is EXISTS, which returns TRUE if the subquery returns at least one row. Often faster than IN for large datasets. A close relative is IN Subquery, which checks if a value matches any value returned by the subquery. More about this category: Nested queries — correlated, EXISTS, IN, ANY, ALL, scalar subqueries. Related Subqueries entries: Correlated Subquery, EXISTS, IN Subquery, ANY / ALL, DERIVED TABLE (Subquery in FROM), LATERAL Subquery (PostgreSQL), Subquery in SELECT, Subquery in HAVING.