Where common table expressions are permitted

Common table expression definitions are permitted in only three places, although they may be referenced throughout the body of the query or in any subqueries.

  • Top-level SELECT statement   Common table expressions are permitted within top-level SELECT statements, but not within subqueries. 
    WITH DepartmentPayroll( DepartmentID, amount ) AS
  ( SELECT DepartmentID, SUM( Salary ) AS amount
    FROM Employees GROUP BY DepartmentID )
SELECT DepartmentID, amount
FROM DepartmentPayroll
WHERE amount = ( SELECT MAX( amount )
              FROM DepartmentPayroll );

  • The top-level SELECT statement in a view definition   Common table expressions are permitted within the top-level SELECT statement that defines a view, but not within subqueries within the definition. 


    CREATE VIEW LargestDept ( DepartmentID, Size, pay ) AS
   WITH
     CountEmployees( DepartmentID, n ) AS
       ( SELECT DepartmentID, COUNT( * ) AS n
         FROM Employees GROUP BY DepartmentID ),
     DepartmentPayroll( DepartmentID, amount ) AS
       ( SELECT DepartmentID, SUM( Salary ) AS amount
         FROM Employees GROUP BY DepartmentID )
  SELECT count.DepartmentID, count.n, pay.amount
  FROM CountEmployees count JOIN DepartmentPayroll pay
  ON count.DepartmentID = pay.DepartmentID
  WHERE count.n = ( SELECT MAX( n ) FROM CountEmployees )
     OR pay.amount = ( SELECT MAX( amount ) FROM DepartmentPayroll );

  • A top-level SELECT statement in an INSERT statement   Common table expressions are permitted within a top-level SELECT statement in an INSERT statement, but not within subqueries within the INSERT statement. 


    CREATE TABLE LargestPayrolls ( DepartmentID INTEGER, Payroll NUMERIC, CurrentDate DATE );
INSERT INTO LargestPayrolls( DepartmentID, Payroll, CurrentDate )
  WITH DepartmentPayroll( DepartmentID, amount ) AS
    ( SELECT DepartmentID, SUM( Salary ) AS amount
      FROM Employees
      GROUP BY DepartmentID )
  SELECT DepartmentID, amount, CURRENT TIMESTAMP
  FROM DepartmentPayroll
  WHERE amount = ( SELECT MAX( amount )
                FROM DepartmentPayroll );

 See also
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