Example: Correlated subquery in a WHERE clause

For each employee in the table, SQL needs to compare the employee's education level to the average education level for the employee's department. In the subquery, you tell SQL to calculate the average education level for the department number in the current row. For example:

  SELECT EMPNO, LASTNAME, WORKDEPT, EDLEVEL
   FROM CORPDATA.EMPLOYEE X
   WHERE EDLEVEL >
      (SELECT AVG(EDLEVEL)
         FROM CORPDATA.EMPLOYEE
         WHERE WORKDEPT = X.WORKDEPT)

A correlated subquery looks like an uncorrelated one, except for the presence of one or more correlated references. In the example, the single correlated reference is the occurrence of X.WORKDEPT in the subselect's FROM clause. Here, the qualifier X is the correlation name defined in the FROM clause of the outer SELECT statement. In that clause, X is introduced as the correlation name of the table CORPDATA.EMPLOYEE.

Now, consider what happens when the subquery is executed for a given row of CORPDATA.EMPLOYEE. Before it is executed, the occurrence of X.WORKDEPT is replaced with the value of the WORKDEPT column for that row. Suppose, for example, that the row is for CHRISTINE I HAAS. Her work department is A00, which is the value of WORKDEPT for this row. The subquery executed for this row is:

  (SELECT AVG(EDLEVEL)
     FROM CORPDATA.EMPLOYEE
     WHERE WORKDEPT = 'A00')

Thus, for the row considered, the subquery produces the average education level of Christine's department. This is then compared in the outer statement to Christine's own education level. For some other row for which WORKDEPT has a different value, that value appears in the subquery in place of A00. For example, for the row for MICHAEL L THOMPSON, this value is B01, and the subquery for his row delivers the average education level for department B01.

The result table produced by the query has the following values: