This document discusses SQL commands for defining and manipulating database tables. It covers creating tables with columns, primary keys, and foreign keys. It also covers SQL commands for data manipulation, including select statements, order by, aggregate functions, group by, subqueries, and joins. It emphasizes the importance of learning SQL through practice and provides references for further reading.

1. Er. Nawaraj Bhandari
Topic 8
SQL 2

2. Data Definition - 1
Create table departments
(dept_no integer not null,
department_name varchar(30),
location varchar2(3)
primary key dept_no);

3. Data Definition - 2
Create table departments
(dept_no integer not null,
department_name varchar(30),
location varchar2(3)
primary key dept_no);
The table name
Here the columns
are defined. The
datatypes
are specified along
with the length in
brackets.
If a column is
specified as NOT
NULL, then it
is mandatory and
must be populated
when a
new row is
created.

4. Primary Key
Create table departments
(dept_no integer not null,
department_name
varchar(30),
location varchar2(3)
primary key dept_no);
Primary Key
defined in
different
places
Create table departments
(dept_no integer not null primary
key,
department_name varchar(30),
location varchar(3));

5. Foreign Key
Create table workers
(emp_no number(5) not null,
first_name varchar(30),
last_name varchar(30),
job_title varchar(30),
age number(3),
dept_no number(5),
primary key emp_no,
foreign key (dept_no) references departments(dept_no));

6. Modifying Tables Using SQL
 Add an extra column
 Drop a column from a table
 Modify the maximum length of the table
 Add a new constraint
 Drop a constraint
 Set a default for a column
 Drop a default for a column

7. Example of Adding a Column Using
Alter Table
ALTER TABLE job_type
ADD salary FLOAT;

8. Data Manipulation
 Select
 Order By
 Aggregate functions
 Group by
 Subqueries
 Joins

9. Select
 SELECT first_name
 FROM Students
 WHERE Student_type = ‘Overseas’;

10. All the Columns or Some of Them
 Select *
 from Students
 Where student_type = ‘Overseas’;
 Select student_id, first_name, last_name,
 From Students
 Where student_type = ‘Overseas’;

11. Order By - 1
 Select first_name, last_name, stu_id
 From Students
 Where student_type = ‘Overseas’
 Order by last_name;

12. Order By - 2
 Select first_name, last_name, stu_id
 From Students
 Where student_type = ‘Overseas’
 Order by last_name desc;

13. Aggregate Functions - 1
 Count
 Sum
 Avg
 Min
 Max

14. Aggregate Functions - 2
 Count – returns number of values in a column
 Sum – returns the sum total of values of a column
 Avg – returns the mean average of values in column
 Min – returns the lowest value in a column
 Max – returns the highest value in a column

15. Example of Aggregate Function
Select branchID, Count(staff_id)
From workers
Where branchType = ‘Main’
Group by branchID
Having Count (staff_id) > 1
Order by branchID
• This counts the number of members of staff in main
branches where there are more than 1 staff member.
It groups them by the branchID.

16. Group By
 As shown it the previous slide...
 This clause is used with an aggregate function and groups the results
by some attribute.

17. Having Clause
 Modifies the group by clause
Select branchID, Count(staff_id)
From workers
Where branchType = ‘Main’
Group by branchID
Having Count (staff_id) > 1
Order by branchID
 In this case, only select groups where it has been calculated by the
count function that there are more than one member of staff.

18. Sub-Queries
Select d.department_name, d.location
From departments d, workers w
Where d.dept_no = w.dept_no
And w.age =
(select max(w2.age)
From workers w2);

19. Joins
Select d.department_name, w.first_name, w.last_name
From departments d, workers w
Where d.dept_no = w.dept_no;

20. Fixing Errors
 Not specifying join condition properly
 Syntax errors
 Spelling errors for keywords
 Columns not existing on tables
 Data-types being mixed up

21. Query Optimisation
 Making sure a query runs as efficiently and as quickly as possible
 Performance
 Examining path a query takes through a database

22. TOAD
 Tools that are used to help optimise queries
 Available at http://www.quest.com/toad/

23. Just Do It!
 Like driving a car...
 The best way to become proficient with SQL is to use it.
 Laboratory sessions
 Assignment

24. Learning Outcomes – Have We Met Them?
 By the end of this topic, students will be able to:
 Understand the syntax of the create statement
 Understand the construction of more complex selections
 Recognise the issues around error messaging and query optimisation

25. References
 Benyon-Davis, P. (2003). Database Systems, 3rd
edition. Palgrave Macmillan. Chapters 11, 12 & 13.
 Connolly, T. & Begg, C. (2004). Database Systems: A
Practical Approach to Design, Implementation, and
Management, 4th Edition. Addison Wesley. Chapters 5,
6 & 7.
 Dietrich, S. W. (2001). Understanding Relational
Database Query Languages, 1st edition. Prentice Hall.
Chapter 5.
 TOAD website http://www.quest.com/toad/

26. ANY QUESTIONS?