Lab 5: Subqueries CISB224 02A, 02B Semester I, 2009/2010 College of Information Technology, Universiti Tenaga Nasional 1

Introduction to Subqueries College of Information Technology, Universiti Tenaga Nasional 2 Suppose that you:  Want to know all the other employees that work in the same department as Zarina Majid  Don’t know Zarina Majid’s department. You need to solve the sub-problem before you can solve the main problem. There are two ways to do this. Let’s call this part the main problem. And this part, the sub-problem.

Introduction to Subqueries – cont. College of Information Technology, Universiti Tenaga Nasional 3 Solution 1 1. Find out the department by executing: SELECT Dept FROM emp WHERE Name = ‘Zarina Majid’ Admin

Introduction to Subqueries – cont. College of Information Technology, Universiti Tenaga Nasional 4 Solution 1 – cont. 2. Then, use the Deparment name obtained to display the other employees: SELECT Name, Title FROM emp WHERE Dept = Admin This method requires you to execute two queries:  One query to solve the sub-problem  Another query to solve the main problem Instead, you could solve the sub-problem and the main problem in just one query.

Introduction to Subqueries – cont. College of Information Technology, Universiti Tenaga Nasional 5 Solution 2 1. Type out the query that solves the main problem. SELECT Name, Title FROM emp WHERE Dept = ‘Admin’

Introduction to Subqueries – cont. College of Information Technology, Universiti Tenaga Nasional 6 Solution 2 – cont. 2. At the search condition, delete the Deparment value and type in a pair of parentheses. SELECT Name, Title FROM emp () WHERE Dept = ()

Introduction to Subqueries – cont. College of Information Technology, Universiti Tenaga Nasional 7 Solution 2 – cont. 3. Within the parentheses, type in the query that solves the sub-problem.

Introduction to Subqueries – cont. College of Information Technology, Universiti Tenaga Nasional 8 Solution 2 – cont. SELECT Name, Title FROM emp WHERE Dept = (SELECT Dept FROM emp WHERE Name = ‘Zarina Majid’ ) Admin

Where You Can Use Subqueries College of Information Technology, Universiti Tenaga Nasional 9  In the WHERE clause  In the HAVING clause  In the FROM clause (later)

Example of Subquery in WHERE College of Information Technology, Universiti Tenaga Nasional 10 Display employees who have the same title as Nordin. SELECT Name, Title, Dept FROM emp WHERE Title = (SELECT Title FROM emp WHERE Name like ‘Nordin%’)

Example of Subquery in HAVING College of Information Technology, Universiti Tenaga Nasional 11 Display all the departments that have an average salary bill greater than the average of Admin deparment

Example of Subquery in HAVING – cont. College of Information Technology, Universiti Tenaga Nasional 12 SELECT Dept, AVG(salary) FROM emp GROUP BY Dept HAVING AVG(salary) < (SELECT AVG(salary) FROM emp WHERE Dept=‘Admin’)

Important!!! College of Information Technology, Universiti Tenaga Nasional 13  You may not have an ORDER BY clause in the subquery  If the subquery returns more than one value i.e. a list of values, than you must use the IN operator in the main query’s search condition

More Operators College of Information Technology, Universiti Tenaga Nasional 14 Operator IS NULL Example: ReportsTo IS NULL IS NOT NULL The negation of the operators that you have learnt NOT BETWEEN NOT IN NOT LIKE

Using More Than One Search Conditions with AND or OR College of Information Technology, Universiti Tenaga Nasional 15 Display the customers who are located in the KL, Perak and Johor SELECT CompName, State FROM cust WHERE State IN (‘KL’, ‘Perak, ‘Johor’ State = ‘KL’ OR State = ‘Perak’ OR State = ‘Johor’