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ZEIT2301
Design of Information Systems
SQL: SELECT Queries
School of Engineering and Information Technology
UNSW@ADFA
Dr Kathryn Merrick
Topic 11: SQL SELECT Queries

Objectives

To introduce the basic notation for writing SELECT queries in SQL
including:
 Query structure
 Wildcards in SELECT
 Mathematical operators in SELECT
 Set membership
Queries in SQL

The most common operation on a database is to query the data
in it

For this reason our main interest will be in SQL queries

There is only one command in SQL for querying data - the
SELECT command
SQL Select Syntax

The basic syntax of the SQL Select command is
SELECT attribute(s)
FROM table(s)
WHERE selection condition(s);
Note:
SQL reserved words are not case sensitive
SQL Select Parameters

A table in a relational database consists of data in rows
(records, tuples) and columns (attributes, fields).

The SELECT clause, with its list of attributes, determines which
columns will be displayed.

The WHERE clause in SQL determines which rows will be
retrieved from the table.
Worked Example
• Given the table:
student (ID, Name, DOB)
ID
833801
930047
957719
834027
927440
940456
954065
950145
954007
Name
Smith
Williams
Marks
Jones
Randall
Fotheringale
Morton
King
Baldwin
DOB
18/03/79
02/05/83
24/09/83
11/06/78
16/06/88
21/12/81
15/10/82
02/01/84
08/04/86
Worked Example (cont.)

Consider the following query:
SELECT Name, DOB
FROM student
WHERE ID = 957719;
Note: quotes not required
for numeric values
-- Attributes
-- Table
-- Rows
Worked Example (cont.)
SELECT Name, DOB
FROM Student
WHERE
ID = 957719
Table: student
ID
833801
930047
957719
834027
927440
940456
954065
950145
954007
Name
Smith
Williams
Marks
Jones
Randall
Fotheringale
Morton
King
Baldwin
DOB
18/03/79
02/05/83
24/09/83
11/06/78
16/06/88
21/12/81
15/10/82
02/01/84
08/04/86
Another Example

Given the table definition:
student (ID, Name, DOB, Address, Gender)
the query:
SELECT ID, Name
FROM student
WHERE DOB > ‘18-Sep-1985’;
Note 1: the date
format varies in
different databases
will fetch from the database, the ID and name of all students born
after (“>”) 18-Sep-1985 (18/9/1985).
Note 2: quotes
required for nonnumeric values
Another Example (cont)
SELECT ID, Name
FROM Student
WHERE
DOB >
‘18-Sep-1985’
Table: student
ID
833801
930047
957719
834027
927440
940456
954065
950145
954007
Name
Smith
Williams
Marks
Jones
Randall
Fotheringale
Morton
King
Baldwin
DOB
18/03/79
02/05/83
24/09/83
11/06/78
16/06/88
21/12/81
15/10/82
02/01/84
08/04/86
In Class Exercise 1

Given the table definition:
student (ID, Name, DOB, Address, Gender)

Write the query:


that will fetch from the database the name and address of all
students born after (>) 20-Sep-1987
Solution:
SELECT Name, Address
FROM student
WHERE DOB > ‘20-Sep-1987’;
Select All Attributes

If all attributes of the target table are required then, rather than
write them all out explicitly, a shorthand is available (“*”):
SELECT *
FROM student
WHERE name = 'Jones';

If all attributes and all rows are required, the FROM clause can be
omitted:
SELECT *
FROM player;
Operations in the WHERE clause

The basic operations that are allowed in the WHERE clause
are:

mathematical operations between appropriate operands: +, -, *, /

relational comparisons: =, >, <, >=, <=, !=

boolean (logical) connectives: and, or, not
Note: Use brackets to define order of
operations , if necessary.
Operator Example

Example: Use of a boolean operator (AND, OR, NOT):
SELECT ID, Name, Address
FROM student
WHERE DOB > ‘18-Sep-1975’ AND
Gender = ‘M’;
In Class Exercise 2

Given the table definition
player (ID, Name, DOB, Address, Gender)

Note: Gender is either
> ‘F’ – for female
> ‘M’ – for male
Write the query:
that will fetch from the database the name, address and DOB of all
female players born during 1985

Solution:
SELECT Name, Address, DOB
FROM player
WHERE Gender = ‘F’ AND
DOB >= '1-Jan-1985' AND DOB <= 31-Dec-1985’;
In Class Exercise 3

Given the table definition:
stock-item (stock-no, description, department-no,
qty-on-hand, buying-price, selling-price, supplier-no)

Write the query:
that lists the stock-numbers for items whose potential profit is over
$500. Write the query:
Hint: Potential profit on an item is:
selling-price – buying-price
Solution
SELECT stock-no
FROM stock-item
WHERE (selling-price – buying-price) > 500;
Note: formatting
characters, such as
“$” not used in query
In Class Exercise 4
Given the table definition:

stock-item (stock-no, description, department-no,
qty-on-hand, buying-price, selling-price, supplier-no)

Write the query:
that lists, for each stock-item, the stock-number and value of
stock on hand (using buying-price as item value)
Hint: SELECT clause can have a calculation in place of a
column name.
buying-price gives the item value; qty-on-hand tells you
how many that item you currently have
Exercise 4 – Solutions
SELECT stock-no, (buying-price * qty-on-hand)
FROM stock-item;
or
SELECT stock-no, (buying-price * qty-on-hand) AS stockValue
FROM stock-item;
Note: In the second solution
the computation is ALIASED
using the AS keyword
Other Operators

Other operators that can be used in the WHERE clause
include:

LIKE - used for wildcard string searches

IS NULL - used to check for null values

BETWEEN - used to define range limits

IN - used to test for membership of a set of values
Wildcards, using LIKE

Wildcard characters may also be used:



“%” matches any string (zero or more characters)
“_” characters, matches exactly one character
Example:
SELECT Number, Name
Note:
MS Access uses * as
the wildcard
FROM student
WHERE Name LIKE ‘Smith%’;

Matches all names that start with ‘Smith’.
Note:
‘like’ for pattern
matching, not “=“
IS NULL

Given the table definition:
stock-item (stock-no, description, department-no,
qty-on-hand, buying-price, selling-price, supplier-no)
SELECT stock-no
FROM stock-item
WHERE supplier-no IS NULL;

Lists the stock-numbers for rows where the
supplier-no is unknown
BETWEEN

Given the table definition:
player (ID, Name, DOB, Address, Height)

Which players are between 180cm and 190cm tall?

Solution:
SELECT Name, Height
FROM player
WHERE Height BETWEEN 180 and 190;
Note:
use the numeric value only (ie 180) not 180cms.
22
Set Membership (IN)

Given the table definition:
player (ID, Name, DOB, Address, Sport)

Which players play racquet sports?

Solution:
SELECT Name
FROM player
WHERE Sport IN (‘Badminton’, ‘Squash’, ‘Tennis’) ;
In Class Exercise 5

Given the table definition:
player (ID, Name, DOB, Grade, Gender)

Write the query:
that will display all details of players in the 1st, 2nd or 3rd grade teams
(More than one solution is valid)
Exercise 5 - Solutions

Solution 1:
Note:
Can’t use: “grade = 1 or 2”,
Must use: “grade = 1 or grade = 2”
SELECT *
FROM player
WHERE grade = 1 OR grade = 2 OR grade = 3;

Solution 2:
Note:
“between” is inclusive in SQL
SELECT *
FROM player
WHERE grade between 1 and 3;

Solution 3:
SELECT *
FROM player
WHERE grade IN (1, 2, 3);
Ordering

Ordering the result of a query is simple - the attribute(s) and
direction (i.e. ASC, DESC) in which the ordering is to be done is
specified after the (optional) ORDER BY clause (default ASC)

Example:
SELECT Number, Name, DOB
FROM student
WHERE DOB > ‘18-Sep-1975’
ORDER BY DOB, Name DESC;
Eliminating Duplicates

By default most SQL implementations will return every
individual row satisfying the query conditions - irrespective of
whether there might be duplicates in the output

Often this is not a problem, but sometimes it can be
inconvenient

In SQL duplicates can be eliminated by use of the DISTINCT
keyword
SQL Select Distinct Example
• Given the table:
student (ID, Name, DOB)
ID
833801
930047
957719
834027
927440
940456
954065
950145
954007
Name
Smith
Williams
Marks
Jones
Randall
Fotheringale
Morton
King
Baldwin
Degree
BA
BSc
MSc
BA
BEng
Msc
BSc
BSc
BA
SQL Select With Duplicates
SELECT Degree
FROM Student
Table: student
ID
833801
930047
957719
834027
927440
940456
954065
950145
954007
Name
Degree
Smith
BA
Williams
BSc
Marks
MSc
Jones
BA
Randall
BEng
Fotheringale
Msc
Morton
BSc
King
BSc
Baldwin
BA
SQL Select Without Duplicates
SELECT DISTINCT Degree
FROM student;

Produces:
Degree
BA
BSc
MSc
BEng
SQL Select Summary
SELECT column1, column2, …
FROM table1
WHERE X = ……..;
-- Attributes
-- Table
-- Rows
Some handy reserved words:
ORDER BY
DISTINCT
LIKE
IS NULL
AND
OR
BETWEEN
IN