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 DeptPayroll( DepartmentID, amt ) AS
  ( SELECT DepartmentID, SUM( Salary ) AS amt
    FROM Employees GROUP BY DepartmentID )
SELECT DepartmentID, amt
FROM DeptPayroll
WHERE amt = ( SELECT MAX( amt )
              FROM DeptPayroll );

  • 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 ),
     DeptPayroll( DepartmentID, amt ) AS
       ( SELECT DepartmentID, SUM( Salary ) AS amt
         FROM Employees GROUP BY DepartmentID )
  SELECT count.DepartmentID, count.n, pay.amt
  FROM CountEmployees count JOIN DeptPayroll pay
  ON count.DepartmentID = pay.DepartmentID
  WHERE count.n = ( SELECT MAX( n ) FROM CountEmployees )
     OR pay.amt = ( SELECT MAX( amt ) FROM DeptPayroll );

  • 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 DeptPayroll( DepartmentID, amt ) AS
    ( SELECT DepartmentID, SUM( Salary ) AS amt
      FROM Employees
      GROUP BY DepartmentID )
  SELECT DepartmentID, amt, CURRENT TIMESTAMP
  FROM DeptPayroll
  WHERE amt = ( SELECT MAX( amt )
                FROM DeptPayroll );

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