SQL Join Basic

Join Operation in
Database
Prepared by: Naimul Arif
Software Engineer
Progoti Systems Ltd.

Simple Review
What is database???
A database is a collection of
information that is organized so that
it can easily be accessed, managed,
and updated.

Why database ???
• To store data
• To retrieve data
• To update data
• To merge data
Where we need database ???
- Every sector where data is handled.

Mostly used databases:
Usage of top 10 databases in August 2014

Types of SQL join operations
• INNER JOIN
• LEFT JOIN
• RIGHT JOIN
• OUTER JOIN
• LEFT JOIN EXCLUDING INNER JOIN
• RIGHT JOIN EXCLUDING INNER JOIN
• OUTER JOIN EXCLUDING INNER JOIN

Before knowing about
SQL join we should know
about Cartesian Product.
Also known as cross join

Cartesian product of tow or more table:
Consider two tables:
PID Pname
1 Shirt
2 Pajabi
3 Lungi
SID ProductID Price
101 1 1000
102 2 800
103 5 400
104 2 600
Product Sale

Cartesian product of tow or more table:
QUERY: Select * from Product, Sale;
PID Pname SID ProductID Price
1 Shirt 101 1 1000
1 Shirt 102 2 800
1 Shirt 103 5 400
1 Shirt 104 2 600
2 Pajabi 101 1 1000
2 Pajabi 102 2 800
2 Pajabi 103 5 400
2 Pajabi 104 2 600
3 Lungi 101 1 1000
3 Lungi 102 2 800
3 Lungi 103 5 400
3 Lungi 104 2 600

Suppose two tables T1 & T1
T1 has r1 rows and c1 columns
T2 has r2 rows and c2 columns
Cartesian product of T1 & T1 will have:
r1 * r2 rows and
c1 + c2 columns.
[for more that two tables r1 * r2 * r3 ...
and c1 + c2 + c3 ...]
Cartesian Product is the all possible
combinations between applied table rows.

PID Pname
1 Shirt
2 Pajabi
3 Lungi
SID ProductID Price
101 1 1000
102 2 800
103 5 400
104 2 600
Product Sale
For ease of learning these two tables
will be used for all examples

Inner join
Query: Select * from Product p Inner join Sale s on p.PID = s.ProductID
Inner join only takes
that rows from
Cartesian Product Table
where the join elements
(Product.PID and
Sale.ProductID in the
above query) matches
fully.

Inner join
1 Shirt 101 1 1000
1 Shirt 102 2 800
1 Shirt 103 5 400
1 Shirt 104 2 600
2 Pajabi 101 1 1000
2 Pajabi 102 2 800
2 Pajabi 103 5 400
2 Pajabi 104 2 600
3 Lungi 101 1 1000
3 Lungi 102 2 800
3 Lungi 103 5 400
3 Lungi 104 2 600

Inner join
1 Shirt 101 1 1000
2 Panjabi 102 2 800
2 Panjabi 104 2 600
So the output of the join table is shown below.
It is to mention that, if there exists more than one matching
element having same value then all possible combination will be
taken.

Left join
Query: Select * from Product p Left join Sale s on p.PID = s.ProductID
Left join takes that rows
which are in inner join output.
And it also looks for the rows
in left table which are not in
inner join output. The rows
are added to OUTPUT with
null in right columns.

Left join
1 Shirt 101 1 1000
1 Shirt 102 2 800
1 Shirt 103 5 400
1 Shirt 104 2 600
2 Pajabi 101 1 1000
2 Pajabi 102 2 800
2 Pajabi 103 5 400
2 Pajabi 104 2 600
3 Lungi 101 1 1000
3 Lungi 102 2 800
3 Lungi 103 5 400
3 Lungi 104 2 600

Left join
1 Shirt 101 1 1000
2 Panjabi 102 2 800
2 Panjabi 104 2 600
3 Lungi null null null
In the left table Product |PID = 3 | Pname = Lungi|
row could not be joined with any row of Sale table. So it is
added with null value in right columns.

Right join
Query: Select * from Product p Right join Sale s on p.PID = s.ProductID
Right join takes that rows
which are in inner join output.
Also looks for the rows in
right table which are not in
null in left columns.

Right join
Query: Select * from Product p Right join Sale s on p.PID = s.ProductID
1 Shirt 101 1 1000
1 Shirt 102 2 800
1 Shirt 103 5 400
1 Shirt 104 2 600
2 Pajabi 101 1 1000
2 Pajabi 102 2 800
2 Pajabi 103 5 400
2 Pajabi 104 2 600
3 Lungi 101 1 1000
3 Lungi 102 2 800
3 Lungi 103 5 400
3 Lungi 104 2 600

Right join
Query: Select * from Product p Right join Sale s on p.PID = p.ProductID
1 Shirt 101 1 1000
2 Panjabi 102 2 800
2 Panjabi 104 2 600
null null 103 5 400
In the right table Sale |SID = 103|ProductID = 5|Price =
400|row could not be joined with any row of Product table. So
it is added with null value in left columns.

Outer join
Query: Select * from Product p Outer join Sale s on p.PID = s.ProductID
Aside of inner join output
Outer join looks for the rows
in left table which are not in
null in right columns. Similarly
the rows from right table not
in inner join output are added
to OUTPUT with null values
in left columns.

Outer join
1 Shirt 101 1 1000
1 Shirt 102 2 800
1 Shirt 103 5 400
1 Shirt 104 2 600
2 Pajabi 101 1 1000
2 Pajabi 102 2 800
2 Pajabi 103 5 400
2 Pajabi 104 2 600
3 Lungi 101 1 1000
3 Lungi 102 2 800
3 Lungi 103 5 400
3 Lungi 104 2 600

Outer join
1 Shirt 101 1 1000
2 Panjabi 102 2 800
2 Panjabi 104 2 600
null null 103 5 400
In the 4th row there is not joinable row in right. So right values
are null. Similarly in 5th row there is no joinable row in left. So
left values are null.

Keep in mind
In case of Cartesian Product there is no matching, only
taking all combination.
In case of Join operation:
 Either matching of two column values are equal.
 Or one of them is null.
 In inner join no null is taken.
 In left join right side null is taken.
 In right join left side null is taken.
 In outer join null can be taken in any side but not in both
side at a time.
 In left join all rows of left table are in output table.
 In right join all rows of right table is taken.
 Number of output rows for join is less or equal to the
number of rows in Cartesian Product.

Left excluding join
Excluding join operations (Left
excluding join, right excluding join
and outer excluding join) only takes
rows which could not be joined. So
strictly one side of the output table
remains null.
Left excluding join operation is nothing but a Left join
operation with a fixed condition.
The condition is: right key should be null.
So eventually all right columns turn null.
Left excluding join = Left join - Inner join

Left excluding join
where s.ProductID is null
PID Pname
1 Shirt
2 Pajabi
3 Lungi
SID ProductID Price
101 1 1000
102 2 800
103 5 400
104 2 600
Product Sale
Here 3rd row of left table has no joinable row in right table.
So output is:

Right excluding join
This operation gives the rows
from right table who have no
joinable row in left table. So left
columns of this join output table
remains null.
Right excluding join operation is nothing but a Right join
operation with a fixed condition.
The condition is: left key should be null.
So eventually all left columns turn null.
Right excluding join = Right join - Inner join

Right excluding join
Query: Select * from Product p Right join Sale s on p.PID = s.ProductID where
p.PID is null
PID Pname
1 Shirt
2 Pajabi
3 Lungi
SID ProductID Price
101 1 1000
102 2 800
103 5 400
104 2 600
Product Sale
Here 3rd row of right table has no joinable row in left table.
So output is:
null null 103 5 400

Outer excluding join
- This operation outputs the rows from
left table with no joinable row in right
table. So right columns are given null.
- Also outputs rows from right table
having no joinable row in left. Left
columns are given null.
- Outer excluding join
= Left outer join + Right outer join
= Outer join - Inner join

Outer excluding join
where Product.PID is or Sale.ProductID is null
PID Pname
1 Shirt
2 Pajabi
3 Lungi
SID ProductID Price
101 1 1000
102 2 800
103 5 400
104 2 600
Product Sale
null null 103 5 400
Lonely row from both table with null to opposite.

Use of join
Join helps to do DB operations keeping tables small and saving
memory. In short normalized database needs join operation.
Example
In facebook there may remain a lot of comments against a single
post. If we keep post and comment info in same table it will look
like this.
Post_
Creator
Post_Time Post_Text Post_ID Comment_
Creator
Comment_
Time
Comment_
Text
Comment_
ID
Sherlock 2014-01-23
00:00:00
I need a case 1001 Moriarty 2014-01-23
00:05:00
Miss me!!! 5001
Sherlock 2014-01-23
00:00:00
I need a case 1001 Joh Watson 2014-01-23
00:06:32
u r a
psychopath!
5002
Sherlock 2014-01-23
00:00:00
I need a case 1001 Sherlock 2014-01-23
00:06:35
nope, i am a
high
functioning
sociopath
5003
Sherlock 2014-01-23
00:00:00
I need a case 1001 Irene Adler 2014-01-23
00:12:01
Let's have
dinner
5004

Use of join
So in the table same post info are inserted a lot of time.
Waste of memory. More comments, more memory waste.
Instead we can maintain three tables, one for Posts, one for
Comments and one to connect them.
The architecture is shown below.
Post CommentHas
Post_Creator PID
CID
Post_Time
Post_Text Post_ID
Comment_
Creator
Comment_
Time
Comment_
Text
Comment_
ID
*

Post_
Creator
Post_Time Post_Text Post_ID
Sherlock 2014-01-23
00:00:00
I need a case 1001
Post_ID Comment_
ID
1001 5001
1001 5002
1001 5003
1001 5004
Comment_
Creator
Comment_
Time
Comment_
Text
Comment_
ID
Moriarty 2014-01-23
00:05:00
Miss me!!! 5001
Joh Watson 2014-01-23
00:06:32
u r a
psychopath!
5002
Sherlock 2014-01-23
00:06:35
nope, i am a
high
functioning
sociopath
5003
Irene Adler 2014-01-23
00:12:01
Let's have
dinner
5004
Post Comment
Has
So memory is optimized.
But what should I do if someone try to view
the post?
Use of join

Use of join
To fetch all comment against a post we need do the
query:
SELECT
p.Post_Creator, p.Post_Time,
p.Post_ID, p.Post_Text,
c.Comment_Creator, c.Comment_Time,
c.Comment_ID, c.Comment_Text
FROM
Post p
INNER JOIN
Has h ON p.Post_ID = h.Post_ID
INNER JOIN
Comment c ON c.Comment_ID = h.Comment_ID
ORDER BY c.Comment_Time

References:
• http://www.codeproject.com/Articles/33052/Vis
ual-Representation-of-SQL-Joins
• http://www.tutorialspoint.com/sql/sql-cartesian-
joins.htm

SQL Join Basic

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