MySQL JOINS Tutorial: 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.

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

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.

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

 

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.

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

 

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

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

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.