SQL

Er. Nawaraj Bhandari
Topic 8
SQL 2

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

Data Definition - 2
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.

Primary Key
department_name
varchar(30),
Primary Key
defined in
different
places
(dept_no integer not null primary
key,
location varchar(3));

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));

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

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

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

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

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’;

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

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

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

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

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.

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

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.

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);

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

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

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

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

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

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

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/

SQL

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