A relational database consists of multiple related tables linking together using common columns which are known as foreign key columns. Because of this, data in each table is incomplete from the business perspective.
For example, in the sample database, we have the
orders and orderdetails tables that are linked using the orderNumber column.
To get complete orders’ data, you need to query data from both
orders and orderdetails table.
And that’s why MySQL
JOIN comes into the play.
A MySQL join is a method of linking data between one (self-join) or more tables based on values of the common column between tables.
First, we create tables like:
| 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 |
MySQL supports the following types of joins:
Cross JOIN
Cross JOIN is the 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 the first table with all records in the 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 a 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 the joined table. It returns NULL values for records of the 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 anyone. 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.
Note: Null is returned for non-matching rows on right
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
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 the 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.
Note: Null is returned for non-matching rows on left
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
"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 the output of each in successive queries. Of course, that is possible. But using JOINs, you can get the work done by using only 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 of 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 subqueries
- 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.
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