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MySQL | INNER JOIN

Database
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MySQL
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Documentation
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Statements and Clauses
schedule Jul 1, 2022
Last updated
local_offer MySQL
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tocTable of Contents
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MySQL's INNER JOIN clause retrieves rows from both tables where there is a common matching column value.

Syntax

SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column = table2.column;

Examples

Consider the following table about some students:

student_id

fname

lname

day_enrolled

age

username

1

Sky

Towner

2015-12-03

17

stowner1

2

Ben

Davis

2016-04-20

19

bdavis2

3

Travis

Apple

2018-08-14

18

tapple3

4

Arthur

David

2016-04-01

16

adavid4

5

Benjamin

Town

2014-01-01

17

btown5

The above sample table can be created using the code here.

Also consider the following table about students' extracurricular activities:

student_id

club

date_entered

1

Football

2016-02-13

2

Boxing

2016-05-25

3

Apple

2018-08-17

4

Fishing

2017-01-01

5

NULL

NULL

The above sample table can be created using the code here.

Basic usage

To retrieve information about students and their extracurricular activities by using the common column student_id:

SELECT fname, lname, age, club
FROM students
INNER JOIN extracurricular
ON students.student_id = extracurricular.student_id;
+----------+--------+------+----------+
| fname | lname | age | club |
+----------+--------+------+----------+
| Sky | Towner | 17 | Football |
| Ben | Davis | 19 | Boxing |
| Travis | Apple | 18 | Chess |
| Arthur | David | 16 | Fishing |
| Benjamin | Town | 17 | NULL |
+----------+--------+------+----------+

Here we return information on fname, lname and age from students table and the corresponding club information from the extracurricular table when there is a match between student_id value in both tables.

Using aliases, the above query can be shortened to:

SELECT fname, lname, age, club
FROM students s
INNER JOIN extracurricular e
ON s.student_id = e.student_id;

Ambiguous column names

The column student_id appears in both the students and extracurricular tables. In such cases if we do not specify which table’s student_id column we are referring to , the confused MySQL server will throw an error:

SELECT student_id, fname, lname, age, club
FROM students
INNER JOIN extracurricular
ON students.student_id = extracurricular.student_id;
ERROR 1052 (23000): Column 'student_id' in field list is ambiguous

The correct way to specify the above if we wanted to retrieve student_id from extracurricular would be:

SELECT extracurricular.student_id, fname, lname, age, club
FROM students
INNER JOIN extracurricular
ON students.student_id = extracurricular.student_id;
+------------+----------+--------+------+----------+
| student_id | fname | lname | age | club |
+------------+----------+--------+------+----------+
| 1 | Sky | Towner | 17 | Football |
| 2 | Ben | Davis | 19 | Boxing |
| 3 | Travis | Apple | 18 | Chess |
| 4 | Arthur | David | 16 | Fishing |
| 5 | Benjamin | Town | 17 | NULL |
+------------+----------+--------+------+----------+

Using aliases the above can be further shortened to:

SELECT e.student_id, fname, lname, age, club
FROM students s
INNER JOIN extracurricular e
ON s.student_id = e.student_id;

We provide students table with an alias s and the extracurricular table with an alias e. This allows us to refer to these tables using the provided aliases elsewhere in the query.

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Published by Arthur Yanagisawa
Edited by 0 others
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