JOINs in MySQL and Other Relational Databases

INNER JOIN (or just JOIN)

SQL INNER JOIN

The most frequently used clause is INNER JOIN. This produces a set of records which match in both the user and course tables, i.e. all users who are enrolled on a course:


SELECT user.name, course.name
FROM `user`
INNER JOIN `course` on user.course = course.id;

LEFT JOIN

SQL LEFT JOIN

What if we require a list of all students and their courses even if they’re not enrolled on one? A LEFT JOIN produces a set of records which matches every entry in the left table (user) regardless of any matching entry in the right table (course):


SELECT user.name, course.name
FROM `user`
LEFT JOIN `course` on user.course = course.id;

RIGHT JOIN

SQL RIGHT JOIN

Perhaps we require a list all courses and students even if no one has been enrolled? A RIGHT JOIN produces a set of records which matches every entry in the right table (course) regardless of any matching entry in the left table (user):


SELECT user.name, course.name
FROM `user`
RIGHT JOIN `course` on user.course = course.id;

OUTER JOIN (or FULL OUTER JOIN)

SQL FULL OUTER JOIN

Our last option is the OUTER JOIN which returns all records in both tables regardless of any match. Where no match exists, the missing side will contain NULL.

OUTER JOIN is less useful than INNER, LEFT or RIGHT and it’s not implemented in MySQL. However, you can work around this restriction using the UNION of a LEFT and RIGHT JOIN, e.g.


SELECT user.name, course.name
FROM `user`
LEFT JOIN `course` on user.course = course.id

UNION

SELECT user.name, course.name
FROM `user`
RIGHT JOIN `course` on user.course = course.id;

 

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