1. CISB224 01A, 01B, 02A, 02B CCSB244 01A, 01B Semester I, 2007/2008
Lab 6: Subqueries
CISB224 01A, 01B, 02A, 02B CCSB244 01A, 01B
Semester I, 2007/2008
College of Information Technology, Universiti Tenaga Nasional

2. Introduction to Subqueries
Suppose that you:
Want to know all the other products supplied by the same company that supplies Outback Lager.
Don’t know who supplies Outback Lager.
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.
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3. Introduction to Subqueries – cont.
Solution 1
Find out who the supplier is by executing:
SELECT SupplierID
FROM Products
WHERE ProductName = ‘Outback Lager’ 7
College of Information Technology, Universiti Tenaga Nasional

4. Introduction to Subqueries – cont.
Solution 1 – cont.
Then, use the Supplier ID obtained to display the other products:
SELECT ProductName
FROM Products
WHERE SupplierID =
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. 7
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5. Introduction to Subqueries – cont.
Solution 2
Type out the query that solves the main problem.
SELECT ProductName
FROM Products
WHERE SupplierID = 7
College of Information Technology, Universiti Tenaga Nasional

6. Introduction to Subqueries – cont.
Solution 2 – cont.
At the search condition, delete the Supplier ID value and type in a pair of parentheses.
SELECT ProductName
FROM Products
WHERE SupplierID = ()
College of Information Technology, Universiti Tenaga Nasional

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

8. Introduction to Subqueries – cont.
Solution 2 – cont.
SELECT ProductName
FROM Products
WHERE SupplierID =
( SELECT SupplierID
FROM Products
WHERE ProductName =
‘Outback Lager’ ) 7
College of Information Technology, Universiti Tenaga Nasional

9. Where You Can Use Subqueries
In the WHERE clause
In the HAVING clause
In the FROM clause (later)
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10. Example of Subquery in WHERE
Display employees who have the same superior as Michael Suyama.
SELECT LastName, ReportsTo
FROM Employees
WHERE ReportsTo = ( SELECT ReportsTo
WHERE LastName = ‘Suyama’)
College of Information Technology, Universiti Tenaga Nasional

11. Example of Subquery in HAVING
Display the number of products supplied by each supplier, but only for suppliers who supply products more than or as many as Supplier 7. Also display the supplier’s company name.
College of Information Technology, Universiti Tenaga Nasional

12. Example of Subquery in HAVING – cont.
SELECTS.SupplierID, CompanyName, Count(ProductID) ‘No. of Products’
FROM Products P JOIN Suppliers S ON P.SupplierID = S.SupplierID
GROUP BY S.SupplierID, CompanyName
HAVING Count(ProductID) >=
(SELECT Count(ProductID)
FROM Products
WHERE SupplierID = 7)
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13. Important!!! 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
College of Information Technology, Universiti Tenaga Nasional

14. More Operators 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
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15. Using More Than One Search Conditions with AND or OR
Display the customers who are located in the North America.
SELECT CompanyName, Country
FROM Customers
WHERE
Country = ‘USA’ OR
Country = ‘Canada’ OR
Country = ‘Mexico’
Country IN (‘USA’, ‘Canada’, ‘Mexico’
College of Information Technology, Universiti Tenaga Nasional

16. Using More Than One Search Conditions with AND or OR – cont.
Display products that are under the Beverages and Seafood category and are not discontinued.
SELECT ProductName
FROM Products
WHERE Discontinued = 0 AND
CategoryID IN ( SELECT CategoryID
FROM Categories
WHERE CategoryName IN
(‘Beverages’, ‘Seafood’))
College of Information Technology, Universiti Tenaga Nasional