Subqueries in SELECT Statements

The following situations define the types of subqueries the database server supports:

• A SELECT statement nested in the Projection list of another SELECT statement
• a SELECT statement nested in the WHERE clause of another SELECT statement (or in an INSERT, DELETE, or UPDATE statement)

Each subquery must contain a Projection clause and a FROM clause. Subqueries can be correlated or uncorrelated. A subquery (or inner SELECT statement) is correlated when the value it produces depends on a value produced by the outer SELECT statement that contains it. For more information, see Correlated Subqueries.Any other kind of subquery is considered uncorrelated.