1. Structured Query Language
Chapter Three
Structured Query Language
Part 2 – Queries

2. Sample DB details PROJECT (ProjectID, Name, Department, MaxHours)
EMPLOYEE (EmployeeNumber, Name, Phone, Department)
ASSIGNMENT (ProjectID, EmployeeNum, HoursWorked)

4. Referential Integrity Constraints for Sample DB
ProjectID in ASSIGNMENT must exist in ProjectID in PROJECT
EmployeeNum in ASSIGNMENT must exist in EmployeeNumber in EMPLOYEE

5. Business Rules for Sample DB
If a PROJECT row is deleted, then all of the ASSIGNMENT rows that are connected to the delete PROJECT row also will be deleted.
If an EMPLOYEE row is deleted and that row is connected to any ASSIGNMENT, the EMPLOYEE row deletion will be disallowed. (It must first be re-assigned to another employee).

6. SQL for Data Retrieval (Queries)
SELECT is the best known SQL statement
SELECT will retrieve information from the database that matches the specified criteria
SELECT EmpName
FROM Emp;

7. The Results of a Query is a Relation
A query pulls information from one or more relations and creates (temporarily) a new relation
This allows for a query to:
Create a new relation
Feed information to another query (as a “sub-query”)

8. Displaying All Columns: *
To show all of the column values for the rows that match the specified criteria, use an *
SELECT *
FROM Emp;

9. Showing a Row Only Once: DISTINCT
A qualifier may be added to the SELECT statement to inhibit duplicate rows from displaying
SELECT DISTINCT DeptID
FROM Emp;

10. Specifying Search Criteria: WHERE
The WHERE clause stipulates the matching criteria for the record that are to be displayed
SELECT EmpName
FROM Emp
WHERE DeptID = 15;

11. Match Criteria The WHERE clause match criteria may include Equals “=“
Not Equals “<>”
Greater than “>”
Less than “<“
Greater than or Equal to “>=“
Less than or Equal to “<=“

12. Match Operators Multiple matching criteria may be specified using AND
Representing an intersection of the data sets OR
Representing a union of the data sets

13. Operator Examples SELECT EmpName FROM Emp WHERE DeptID < 7
OR DeptID > 12;

14. Operator Examples SELECT EmpName FROM Emp WHERE DeptID = 9
AND SalaryCode <= 23;

15. A List of Values
The WHERE clause may specify that a particular column value must be included in a list of values
SELECT EmpName
FROM Emp
WHERE DeptID IN (4, 8, 9);

16. The Logical NOT Operator
Any criteria statement may be preceded by a NOT operator which is to say that all information will be shown except that information matching the specified criteria
SELECT EmpName
FROM Emp
WHERE DeptID NOT IN (4, 8, 9);

17. Finding Data Matching a Range of Values: BETWEEN
SQL provides a BETWEEN statement that allows a user to specify a minimum and maximum value on one line
SELECT EmpName
FROM Emp
WHERE SalaryCode BETWEEN 10 AND 45;

18. Allowing for Wildcard Searches: LIKE
Sometimes it may be advantageous to find rows matching a string value using wildcards
Single character wildcard character is an underscore (_)
Multiple character wildcard character is a percent sign (%)

19. Wildcard Search Examples
SELECT EmpID
FROM Emp
WHERE EmpName LIKE ‘Kr%’;
SELECT EmpID
WHERE Phone LIKE ‘616-___-____’;

20. Sorting the Results: ORDER BY
The results may be sorted using the ORDER BY clause
SELECT *
FROM Emp
ORDER BY EmpName;

21. Built-in SQL Functions
SQL provides several built-in functions
COUNT
Counts the number of rows that match the specified criteria
MIN
Finds the minimum value for a specific column for those rows matching the criteria
MAX
Finds the maximum value for a specific column for those rows matching the criteria

22. Built-in SQL Functions (continued)
SUM
Calculates the sum for a specific column for those rows matching the criteria
AVG
Calculates the numerical average of a specific column for those rows matching the criteria

23. Built-in Function Examples
SELECT COUNT(DISTINCT DeptID)
FROM Emp;
SELECT MIN(Hours), MAX(Hours), AVG(Hours)
FROM Project
WHERE ProjID > 7;

24. Providing Subtotals: GROUP BY
Subtotals may be calculated by using the GROUP BY clause
SELECT DeptID, COUNT(*)
FROM Emp
GROUP BY DeptID
HAVING Count(*) > 3;

25. Retrieving Information from Multiple Tables
SubQueries
As stated earlier, the result of a query is a relation. As a result, a query may feed another query. This is called a subquery
Joins
Another way of combining data is by using a Join
Join [also called an Inner Join]
Left Outer Join
Right Outer Join

26. Subquery SELECT EmpName FROM Emp WHERE DeptID in (SELECT DeptID
FROM Department
WHERE DeptName
LIKE ‘Accounts%’);

27. Join SELECT EmpName FROM Emp, Department
WHERE Emp.DeptID = Department.DeptID
AND Department.DeptName
LIKE ‘Account%’;

28. Left Outer Join Left Outer Join SELECT EmpName
FROM Emp LEFT JOIN Department
WHERE Emp.DeptID = Department.DeptID
AND Department.DeptName
LIKE ‘Account%’;

29. Right Outer Join Right Outer Join SELECT EmpName
FROM Department RIGHT JOIN Emp
WHERE Emp.DeptID = Department.DeptID
AND Department.DeptName
LIKE ‘Account%’;