MySQL JOINS: INNER, OUTER, LEFT, RIGHT, CROSS

 What are JOINS?

Joins help retrieving data from two or more database tables. 

The tables are mutually related using primary and foreign keys.

Note: JOIN is the most misunderstood topic amongst SQL leaners. For sake of simplicity and ease of understanding , we will be using a new Database to practice sample.  As shown below

id	first_name	last_name	movie_id
1	Adam	Smith	1
2	Ravi	Kumar	2
3	Susan	Davidson	5
4	Jenny	Adrianna	8
6	Lee	Pong	10

id	title	category
1	ASSASSIN'S CREED: EMBERS	Animations
2	Real Steel(2012)	Animations
3	Alvin and the Chipmunks	Animations
4	The Adventures of Tin Tin	Animations
5	Safe (2012)	Action
6	Safe House(2012)	Action
7	GIA	18+
8	Deadline 2009	18+
9	The Dirty Picture	18+
10	Marley and me	Romance

 

Types of joins

 
Cross JOIN
Cross JOIN is a simplest form of JOINs which matches each row from one database table to all rows of another.
In other words it gives us combinations of each row of first table with all records in second table.
Suppose we want to get all member records against all the movie records, we can use the script shown below to get our desired results.

SELECT * FROM `movies` CROSS JOIN `members`

Executing the above script in MySQL workbench gives us the following results.
 

id
title
 
id
first_name
last_name
movie_id
1
ASSASSIN'S CREED: EMBERS
Animations
1
Adam
Smith
1
1
ASSASSIN'S CREED: EMBERS
Animations
2
Ravi
Kumar
2
1
ASSASSIN'S CREED: EMBERS
Animations
3
Susan
Davidson
5
1
ASSASSIN'S CREED: EMBERS
Animations
4
Jenny
Adrianna
8
1
ASSASSIN'S CREED: EMBERS
Animations
6
Lee
Pong
10
2
Real Steel(2012)
Animations
1
Adam
Smith
1
2
Real Steel(2012)
Animations
2
Ravi
Kumar
2
2
Real Steel(2012)
Animations
3
Susan
Davidson
5
2
Real Steel(2012)
Animations
4
Jenny
Adrianna
8
2
Real Steel(2012)
Animations
6
Lee
Pong
10
3
Alvin and the Chipmunks
Animations
1
Adam
Smith
1
3
Alvin and the Chipmunks
Animations
2
Ravi
Kumar
2
3
Alvin and the Chipmunks
Animations
3
Susan
Davidson
5
3
Alvin and the Chipmunks
Animations
4
Jenny
Adrianna
8
3
Alvin and the Chipmunks
Animations
6
Lee
Pong
10
4
The Adventures of Tin Tin
Animations
1
Adam
Smith
1
4
The Adventures of Tin Tin
Animations
2
Ravi
Kumar
2
4
The Adventures of Tin Tin
Animations
3
Susan
Davidson
5
4
The Adventures of Tin Tin
Animations
4
Jenny
Adrianna
8
4
The Adventures of Tin Tin
Animations
6
Lee
Pong
10
5
Safe (2012)
Action
1
Adam
Smith
1
5
Safe (2012)
Action
2
Ravi
Kumar
2
5
Safe (2012)
Action
3
Susan
Davidson
5
5
Safe (2012)
Action
4
Jenny
Adrianna
8
5
Safe (2012)
Action
6
Lee
Pong
10
6
Safe House(2012)
Action
1
Adam
Smith
1
6
Safe House(2012)
Action
2
Ravi
Kumar
2
6
Safe House(2012)
Action
3
Susan
Davidson
5
6
Safe House(2012)
Action
4
Jenny
Adrianna
8
6
Safe House(2012)
Action
6
Lee
Pong
10
7
GIA
18+
1
Adam
Smith
1
7
GIA
18+
2
Ravi
Kumar
2
7
GIA
18+
3
Susan
Davidson
5
7
GIA
18+
4
Jenny
Adrianna
8
7
GIA
18+
6
Lee
Pong
10
8
Deadline(2009)
18+
1
Adam
Smith
1
8
Deadline(2009)
18+
2
Ravi
Kumar
2
8
Deadline(2009)
18+
3
Susan
Davidson
5
8
Deadline(2009)
18+
4
Jenny
Adrianna
8
8
Deadline(2009)
18+
6
Lee
Pong
10
9
The Dirty Picture
18+
1
Adam
Smith
1
9
The Dirty Picture
18+
2
Ravi
Kumar
2
9
The Dirty Picture
18+
3
Susan
Davidson
5
9
The Dirty Picture
18+
4
Jenny
Adrianna
8
9
The Dirty Picture
18+
6
Lee
Pong
10
10
Marley and me
Romance
1
Adam
Smith
1
10
Marley and me
Romance
2
Ravi
Kumar
2
10
Marley and me
Romance
3
Susan
Davidson
5
10
Marley and me
Romance
4
Jenny
Adrianna
8
10
Marley and me
Romance
6
Lee
Pong
10

 

INNER JOIN

The inner JOIN is used to return rows from both tables that satisfy the given condition.
Suppose , you want to get list of members who have rented movies together with titles of movies rented by them. You can simply use an INNER JOIN for that, which returns rows from both tables that satisfy with given conditions.

 

SELECT members.`first_name` , members.`last_name` , movies.`title`
FROM members ,movies
WHERE movies.`id` = members.`movie_id`

Executing the above script give

 

first_name
last_name
title
Adam
Smith
ASSASSIN'S CREED: EMBERS
Ravi
Kumar
Real Steel(2012)
Susan
Davidson
Safe (2012)
Jenny
Adrianna
Deadline(2009)
Lee
Pong
Marley and me

Note the above results script can also be written as follows to achieve the same results.

SELECT A.`first_name` , A.`last_name` , B.`title`
FROM `members`AS A
INNER JOIN `movies` AS B
ON B.`id` = A.`movie_id`

Outer JOINs

MySQL Outer JOINs return all records matching from both tables .
It can detect records having no match in joined table. It returns NULL values for records of joined table if no match is found.
Sounds Confusing ? Let's look into an example -

LEFT JOIN

Assume now you want to get titles of all movies together with names of members who have rented them. It is clear that some movies have not being rented by any one. We can simply use LEFT JOIN for the purpose.

 
The LEFT JOIN returns all the rows from the table on the left even if no matching rows have been found in the table on the right. Where no matches have been found in the table on the right, NULL is returned.
 

SELECT A.`title` , B.`first_name` , B.`last_name`
FROM `movies` AS A
LEFT JOIN `members` AS B
ON B.`movie_id` = A.`id`

Executing the above script in MySQL workbench gives.You can see that in the returned result which is listed below that for movies which are not rented, member name fields are having NULL values. That means no matching member found members table for that particular movie.

 

title
first_name
last_name
ASSASSIN'S CREED: EMBERS
Adam
Smith
Real Steel(2012)
Ravi
Kumar
Safe (2012)
Susan
Davidson
Deadline(2009)
Jenny
Adrianna
Marley and me
Lee
Pong
Alvin and the Chipmunks
NULL
NULL
The Adventures of Tin Tin
NULL
NULL
Safe House(2012)
NULL
NULL
GIA
NULL
NULL
The Dirty Picture
NULL
NULL

Note: Null is returned for non-matching rows on right

 

RIGHT JOIN

RIGHT JOIN is obviously the opposite of LEFT JOIN. The RIGHT JOIN returns all the columns from the table on the right even if no matching rows have been found in the table on the left. Where no matches have been found in the table on the left, NULL is returned.
In our example,  let's assume that you need to get names of members and movies rented by them. Now we have a new member who has not rented any movie yet

SELECT  A.`first_name` , A.`last_name`, B.`title`
FROM `members` AS A
RIGHT JOIN `movies` AS B
ON B.`id` = A.`movie_id`

 
Executing the above script in MySQL workbench gives the following results.

first_name
last_name
title
Adam
Smith
ASSASSIN'S CREED: EMBERS
Ravi
Kumar
Real Steel(2012)
Susan
Davidson
Safe (2012)
Jenny
Adrianna
Deadline(2009)
Lee
Pong
Marley and me
NULL
NULL
Alvin and the Chipmunks
NULL
NULL
The Adventures of Tin Tin
NULL
NULL
Safe House(2012)
NULL
NULL
GIA
NULL
NULL
The Dirty Picture

Note: Null is returned for non-matching rows on left

"ON" and "USING" clauses

In above JOIN query examples, we have used ON clause to match the records between table.
USING clause can also be used for the same purpose. The difference with USING is it needs to have identical names for matched columns in both tables.
 
In "movies" table so far we used its primary key with the name "id". We referred to same in  "members" table with the name "movie_id".
 
Let's rename "movies" tables "id" field to have the name "movie_id". We do this in order to have identical matched field names.

ALTER TABLE `movies` CHANGE `id` `movie_id` INT( 11 ) NOT NULL AUTO_INCREMENT;

Next let's use USING with above LEFT JOIN example.

SELECT A.`title` , B.`first_name` , B.`last_name`
FROM `movies` AS A
LEFT JOIN `members` AS B
USING ( `movie_id` )

Apart from using ON and USING with JOINs you can use many other MySQL clauses like GROUP BY, WHERE and even functions like SUM, AVG, etc.
 

Why should we use joins?

Now you may think, why we use JOINs when we can do the same task running queries. Especially if you have some experience in database programming you know we can run queries one by one, use output of each in successive queries. Of course, that is possible. But using JOINs, you can get the work done by using only a one query with any search parameters. On the other hand MySQL can achieve better performance with JOINs as it can use Indexing. Simply use of single JOIN query instead running multiple queries do reduce server overhead. Using multiple queries instead that leads more data transfers between MySQL and applications (software). Further it requires more data manipulations in application end also.
It is clear that we can achieve better MySQL and application performances by use of JOINs.
 

Summary

• JOINS allow us to combine data from more than one table into a single result set.
• JOINS have better performance compared to sub queries
• INNER JOINS only return rows that meet the given criteria.
• OUTER JOINS can also return rows where no matches have been found. The unmatched rows are returned with the NULL keyword.
• The major JOIN types include Inner, Left Outer, Right Outer, Cross JOINS etc.
• The frequently used clause in JOIN operations is "ON". "USING" clause requires that matching columns be of the same name.
• JOINS can also be used in other clauses such as GROUP BY, WHERE, SUB QUERIES, AGGREGATE FUNCTIONS etc.