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CpSc 3220
The Language of SQL
The Language of SQL
Chapters 13-14
Topics
• Self Joins and Views
• Subqueries
• Set Logic
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
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
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
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
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
SELECT
ID,name,Instructor.dept_name,building,budget
FROM InstructorData
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 ;
Benefits of Views
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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
Modifying and Deleting Views
Views can be changed with the ALTER VIEW and
DROP VIEW statements
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
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
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)
Subqueries in Selection Criteria
SELECT CustomerName
FROM Customers
WHERE CustomerID IN
(SELECT CustomerID
FROM Orders
WHERE OrderType = ‘Cash’);
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;
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);
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
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’