SQL SELECT

In this Lecture
SQL SELECT
WHERE clauses
SELECT from multiple tables
JOINs
For more information
Connolly and Begg Chapter 5
Ullman and Widom Chapter 6.1-6.3

SQL SELECT Overview
SELECT
[DISTINCT | ALL]
FROM
[WHERE
]
[ORDER BY
]
[GROUP BY
]
[HAVING
]
([]- optional, | - or)

Example Tables

DISTINCT and ALL
Sometimes you end up with duplicate entries
Using DISTINCT removes duplicates
Using ALL retains them - this is the default
SELECT ALL Last
FROM Student
SELECT DISTINCT Last
FROM Student

WHERE Clauses
Usually you don’t want all the rows
A WHERE clause restricts the rows that are returned
It takes the form of a condition - only those rows that satisfy the condition are returned
Example conditions:
Mark < 40
First = ‘John’
First <> ‘John’
First = Last
(First = ‘John’) AND
(Last = ‘Smith’)
(Mark < 40) OR (Mark > 70)

WHERE Examples
SELECT * FROM Grade
WHERE Mark >= 60
SELECT DISTINCT ID
FROM Grade
WHERE Mark >= 60
ID Code Mark
S103 DBS 72
S104 PR1 68
S104 IAI 65
S107 PR1 76
S107 PR2 60

WHERE Example
Given the table
Write an SQL query to find a list of the ID numbers and marks in IAI of students who have passed (scored 40 or higher) IAI
ID Mark
S103 58
S104 65

One Solution
SELECT ID, Mark FROM Grade
WHERE (Code = ‘IAI’) AND
(Mark >= 40)

SELECT from Multiple Tables
Often you need to combine information from two or more tables
You can get the effect of a product by using
SELECT * FROM Table1, Table2...
If the tables have columns with the same name ambiguity results
You resolve this by referencing columns with the table name
TableName.Column

SELECT from Multiple Tables
SELECT
First, Last, Mark
FROM Student, Grade
WHERE
(Student.ID =
Grade.ID) AND
(Mark >= 40)

SELECT from Multiple Tables
ID First Last ID Code Mark
S103 John Smith S103 DBS 72
S103 John Smith S103 IAI 58
S103 John Smith S104 PR1 68
S103 John Smith S104 IAI 65
S103 John Smith S106 PR2 43
S103 John Smith S107 PR1 76
S103 John Smith S107 PR2 60
S103 John Smith S107 IAI 35
S104 Mary Jones S103 DBS 72
S104 Mary Jones S103 IAI 58
S104 Mary Jones S104 PR1 68
S104 Mary Jones S104 IAI 65
S104 Mary Jones S106 PR2 43
SELECT ... FROM Student, Grade WHERE...

SELECT from Multiple Tables
ID First Last ID Code Mark
S103 John Smith S103 DBS 72
S103 John Smith S103 IAI 58
S104 Mary Jones S104 PR1 68
S104 Mary Jones S104 IAI 65
S106 Mark Jones S106 PR2 43
S107 John Brown S107 PR1 76
S107 John Brown S107 PR2 60
S107 John Brown S107 IAI 35
SELECT ... FROM Student, Grade
WHERE (Student.ID = Grade.ID) AND ...

SELECT from Multiple Tables
ID First Last ID Code Mark
S103 John Smith S103 DBS 72
S103 John Smith S103 IAI 58
S104 Mary Jones S104 PR1 68
S104 Mary Jones S104 IAI 65
S106 Mark Jones S106 PR2 43
S107 John Brown S107 PR1 76
S107 John Brown S107 PR2 60
SELECT ... FROM Student, Grade
WHERE (Student.ID = Grade.ID) AND (Mark >= 40)

SELECT from Multiple Tables
First Last Mark
John Smith 72
John Smith 58
Mary Jones 68
Mary Jones 65
Mark Jones 43
John Brown 76
John Brown 60
SELECT First, Last, Mark FROM Student, Grade
WHERE (Student.ID = Grade.ID) AND (Mark >= 40)

SELECT from Multiple Tables
When selecting from multiple tables you almost always use a WHERE clause to find entries with common values
SELECT * FROM
Student, Grade, Course
WHERE
Student.ID = Grade.ID
AND
Course.Code =
Grade.Code

SELECT from Multiple Tables
Student
Grade
Course
Student.ID = Grade.ID
Course.Code = Grade.Code

JOINs
JOINs can be used to combine tables
There are many types of JOIN
CROSS JOIN
INNER JOIN
NATURAL JOIN
OUTER JOIN
OUTER JOINs are linked with NULLs - more later
A CROSS JOIN B
returns all pairs of rows from A and B
A NATURAL JOIN B
returns pairs of rows with common values for identically named columns and without duplicating columns
A INNER JOIN B
returns pairs of rows satisfying a condition

CROSS JOIN
SELECT * FROM
Student CROSS JOIN
Enrolment
ID Name ID Code
123 John 123 DBS
124 Mary 123 DBS
125 Mark 123 DBS
126 Jane 123 DBS
123 John 124 PRG
124 Mary 124 PRG
125 Mark 124 PRG
126 Jane 124 PRG
123 John 124 DBS
124 Mary 124 DBS

NATURAL JOIN
SELECT * FROM
Student NATURAL JOIN Enrolment
Code
DBS
PRG
DBS
PRG

CROSS and NATURAL JOIN
SELECT * FROM
A CROSS JOIN B
is the same as
SELECT * FROM A, B
SELECT * FROM
A NATURAL JOIN B
is the same as
SELECT A.col1,… A.coln, [and all other columns apart from B.col1,…B.coln]
FROM A, B
WHERE A.col1 = B.col1
AND A.col2 = B.col2
...AND A.coln = B.col.n
(this assumes that col1… coln in A and B have common names)

INNER JOIN
INNER JOINs specify a condition which the pairs of rows satisfy
SELECT * FROM
A INNER JOIN B
ON
Can also use
SELECT * FROM
A INNER JOIN B
USING
(col1, col2,…)
Chooses rows where the given columns are equal

INNER JOIN
SELECT * FROM
Student INNER JOIN Enrolment USING (ID)
ID Name ID Code
123 John 123 DBS
124 Mary 124 PRG
124 Mary 124 DBS
126 Jane 126 PRG

INNER JOIN
SELECT * FROM
Buyer INNER JOIN Property ON
Price <= Budget
Name Budget Address Price
Smith 100,000 15 High St 85,000
Jones 150,000 15 High St 85,000
Jones 150,000 12 Queen St 125,000

INNER JOIN
SELECT * FROM
A INNER JOIN B
ON
is the same as
SELECT * FROM A, B
WHERE
SELECT * FROM
A INNER JOIN B
USING(col1, col2,...)
is the same as
SELECT * FROM A, B
WHERE A.col1 = B.col1
AND A.col2 = B.col2
AND ...

JOINs vs WHERE Clauses
JOINs (so far) are not needed
You can have the same effect by selecting from multiple tables with an appropriate WHERE clause
So should you use JOINs or not?
Yes, because
They often lead to concise queries
NATURAL JOINs are very common
No, because
Support for JOINs varies a fair bit among SQL dialects

Writing Queries
When writing queries
There are often many ways to write the query
You should worry about being correct, clear, and concise in that order
Don’t worry about being clever or efficient
Most DBMSs have query optimisers
These take a user’s query and figure out how to efficiently execute it
A simple query is easier to optimise
We’ll look at some ways to improve efficiency later

This Lecture in Exams

This Lecture in Exams
Find a list of all the CD titles.
(1 mark)
Find a list of the titles of tracks that are more than 300 seconds long.
(2 marks)
Find a list of the names of those artists who have a track on the CD with the title “Compilation”.
(4 marks)