Subqueries in SELECT statements

In simple WHERE and HAVING clauses, you can specify a search condition by using a literal value, a column name, an expression, or a special register. In those search conditions, you know that you are searching for a specific value. However, sometimes you cannot supply that value until you have retrieved other data from a table. For example, suppose you want a list of the employee numbers, names, and job codes of all employees working on a particular project, say project number MA2100. The first part of the statement is easy to write:

     SELECT EMPNO, LASTNAME, JOB
     FROM CORPDATA.EMPLOYEE
     WHERE EMPNO …

But you cannot go further because the CORPDATA.EMPLOYEE table does not include project number data. You do not know which employees are working on project MA2100 without issuing another SELECT statement against the CORPDATA.EMP_ACT table.

With SQL, you can nest one SELECT statement within another to solve this problem. The inner SELECT statement is called a subquery. The SELECT statement surrounding the subquery is called the outer-level SELECT. Using a subquery, you can issue just one SQL statement to retrieve the employee numbers, names, and job codes for employees who work on project MA2100:

     SELECT EMPNO, LASTNAME, JOB
     FROM CORPDATA.EMPLOYEE
     WHERE EMPNO IN
        (SELECT EMPNO
           FROM CORPDATA.EMPPROJACT
           WHERE PROJNO = 'MA2100')

To better understand what will result from this SQL statement, imagine that SQL goes through the following process:

Step 1: SQL evaluates the subquery to obtain a list of EMPNO values:

(SELECT EMPNO
     FROM CORPDATA.EMPPROJACT
     WHERE PROJNO= 'MA2100')

Which results in an interim results table:

Step 2: The interim results table then serves as a list in the search condition of the outer-level SELECT. Essentially, this is the statement that is run.

SELECT EMPNO, LASTNAME, JOB
     FROM CORPDATA.EMPLOYEE
     WHERE EMPNO IN 
            ('000010', '000110')

The final result table looks like this: