Using WITH clause to simplify complex SQL in Oracle

The WITH clause, or subquery factoring clause, is part of the SQL-99 standard and was added into the Oracle SQL syntax in Oracle 9.2. The WITH clause may be processed as an inline view or resolved as a temporary table. The advantage of the latter is that repeated references to the subquery may be more efficient as the data is easily retrieved from the temporary table, rather than being requeried by each reference.

The following example shows how the WITH clause can be used to reduce repetition and simplify complex SQL statements.

Using the SCOTT schema, for each employee we want to know how many other people are in their department. Using an inline view we might do the following.

SELECT e.ename AS employee_name,
       dc.dept_count AS emp_dept_count
FROM   emp e,
       (SELECT deptno, COUNT(*) AS dept_count
        FROM   emp
        GROUP BY deptno) dc
WHERE  e.deptno = dc.deptno;

Using a WITH clause this would look like the following.

WITH dept_count AS (
  SELECT deptno, COUNT(*) AS dept_count
  FROM   emp
  GROUP BY deptno)
SELECT e.ename AS employee_name,
       dc.dept_count AS emp_dept_count
FROM   emp e,
       dept_count dc
WHERE  e.deptno = dc.deptno;

What if we also want to pull back each employees manager name and the number of people in the managers department? Using the inline view it now looks like this.

SELECT e.ename AS employee_name,
       dc1.dept_count AS emp_dept_count,
       m.ename AS manager_name,
       dc2.dept_count AS mgr_dept_count
FROM   emp e,
       (SELECT deptno, COUNT(*) AS dept_count
        FROM   emp
        GROUP BY deptno) dc1,
       emp m,
       (SELECT deptno, COUNT(*) AS dept_count
        FROM   emp
        GROUP BY deptno) dc2
WHERE  e.deptno = dc1.deptno
AND    e.mgr = m.empno
AND    m.deptno = dc2.deptno;

Using the WITH clause this would look like the following.

WITH dept_count AS (
  SELECT deptno, COUNT(*) AS dept_count
  FROM   emp
  GROUP BY deptno)
SELECT e.ename AS employee_name,
       dc1.dept_count AS emp_dept_count,
       m.ename AS manager_name,
       dc2.dept_count AS mgr_dept_count
FROM   emp e,
       dept_count dc1,
       emp m,
       dept_count dc2
WHERE  e.deptno = dc1.deptno
AND    e.mgr = m.empno
AND    m.deptno = dc2.deptno;

So we don't need to redefine the same subquery multiple times. Instead we just use the query name defined in the WITH clause, making the query much easier to read.

If the contents of the WITH clause is sufficiently complex, Oracle may decide to resolve the result of the subquery into a global temporary table. This can make multiple references to the subquery more efficient. The MATERIALIZE and INLINE optimizer hints can be used to influence the decision. The undocumented MATERIALIZE hint tells the optimizer to resolve the subquery as a global temporary table, while the INLINE hint tells it to process the query inline.

WITH dept_count AS (
  SELECT /*+ MATERIALIZE */ deptno, COUNT(*) AS dept_count
  FROM   emp
  GROUP BY deptno)
SELECT ...

WITH dept_count AS (
  SELECT /*+ INLINE */ deptno, COUNT(*) AS dept_count
  FROM   emp
  GROUP BY deptno)
SELECT ...