1. CpSc 3220 The Language of SQL
Chapters 13-14

2. Topics
Self Joins and Views
Subqueries
Set Logic

3. Self Joins A self join is the join of a table to itself
Many tables are self-referencing
Examples:
A Personnel table contains a column that gives the ID of an employee’s manager who is also a employee in the Personnel table
A Course table might contain a column that gives its prerequisite which is also an entry in the Course table

4. Implementing Self Joins
Use the INNER JOIN to join a table to itself
The AS word must be used to give each table a distinct name so that references to columns can be made unique
Example:
SELECT E.EmployeeName,M.EmployeeName
FROM Personnel AS E
INNER JOIN Personnel as M
ON E.ManagerID = M.EmployeeID

5. Creating Views
A View is a virtual table that can be saved and used in queries like a normal table This can be used to simplify other queries For example, if we have several queries to make on joined tables we can create a View of the joined tables and then use that virtual table directly for all the queries

6. Example of View Creation
From the University Database we can create a View called InstructorData that contains information about an Instructor and the Department the instructor is assigned to
CREATE VIEW InstructorData AS
SELECT
ID,name,Instructor.dept_name,salary,building,budget
FROM Instructor INNER JOIN Department
ON Instructor.dept_name = Department.dept_name
Views can be used in queries as can Tables

7. Query from A View Views can be used in queries just as can Tables
SELECT
ID,name,Instructor.dept_name,building
FROM Instructor INNER JOIN Department
ON Instructor.dept_name = Department.dept_name
ID,name,Instructor.dept_name,building,budget
FROM InstructorData

8. Views Can Limit Access to Data
From the University Database we can create a View called PublicInstructorData that contains non-sensitive information about an Instructor
CREATE VIEW PublicInstructorData AS
SELECT
ID,name,dept_name
FROM Instructor ;

9. Benefits of Views Can reduce complexity Can increase reusability
Can format data
Can create calculated columns
Can rename columns
Can create subsets of data
Can enforce security restrictions

10. Modifying and Deleting Views
Views can be changed with the ALTER VIEW and DROP VIEW statements

11. Subqueries
It is possible for queries to contain other queries Using nested queries can be complex Can be used in SELECT, INSERT, UPDATE, and DELETE statements

12. Types of Subqueries General form of SELECT statement
SELECT colList
FROM tableList
WHERE condition
GROUP BY colList
HAVING condition
ORDER BY colList
Subqueries can be used in colList, tableList, or condition sections

13. Subqueries as Data Sources
An example from University Database
We want a list of students and the courses they took in 2012
SELECT name,course_id
FROM Student
WHERE Student.ID IN
(SELECT Takes.ID
FROM Takes
WHERE Takes.ID = Student.ID)

14. Subqueries in Selection Criteria
SELECT CustomerName FROM Customers WHERE CustomerID IN (SELECT CustomerID FROM Orders WHERE OrderType = ‘Cash’);

15. Correlated Subqueries
Related to the outer query
Must be evaluated for each returned row
Example:
SELECT CustomerName
FROM Customers AS C WHERE
(SELECT SUM(OrderAmount)
FROM Orders AS O
WHERE C.CustomerID=O.CustomerID) > 20;

16. The EXISTS Operator
Allows you to determine if data in a correlated subquery exists
SELECT CustomerName
FROM Customers AS C
WHERE EXISTS
(SELECT * FROM ORDERS
WHERE C.CustomerID=O.CustomerID);

17. Subqueries as Calculated Columns
SELECT CustomerName, (SELECT COUNT(OrderID) FROM Orders WHERE Customers.CustomerID=Orders.CustomerID)AS ‘No.of Orders) FROM Customers ORDER BY Customers.CustomerID

18. Set Logic
Remember, Relational Databases are based on the assumption that the Tables are really mathematical relations (or sets)
We can combine queries using Set Logic (UNION, INTERSECT, DIFFERENCE)
select-statement1 UNION select-statement2
Combined relations must be of the same ‘type’