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MySQL | UNION

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 UNION clause combines the results from multiple SELECT statements into a single result set.

In order to perform a UNION on tables, they must first satisfy the following conditions:

• Same number of columns

• Corresponding columns must have compatible data types

Syntax

SELECT column(s) FROM table1
UNION
SELECT column(s) FROM table2;

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 some professors:

professor_id

fname

lname

day_hired

age

username

1

Tom

Willox

2012-11-03

47

twillox1

2

Ralph

Skeen

2009-07-30

39

rskeen2

3

Hyun

Lee

2014-02-21

32

hlee3

4

Bryce

Miller

2004-10-01

53

bmiller4

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

Basic usage

To return a list of the names of all students and professors at the school:

SELECT fname, lname FROM students
UNION
SELECT fname, lname FROM professors;
+----------+--------+
| fname | lname |
+----------+--------+
| Sky | Towner |
| Ben | Davis |
| Travis | Apple |
| Arthur | David |
| Benjamin | Town |
| Tom | Willox |
| Ralph | Skeen |
| Hyun | Lee |
| Bryce | Miller |
+----------+--------+

We can see that the results from the individual SELECT statements have been combined into one result set. In this way we are able to see the records from students and professors tables together.

NOTE

By default, duplicates are removed from the union set.( UNION DISTINCT is the same as just UNION). UNION ALL will allow for duplicates.

Different number of columns

We can only perform UNION when the SELECT statements involved return the same number of columns:

SELECT fname, lname, username FROM students
UNION
SELECT fname, lname FROM professors;
ERROR 1222 (21000): The used SELECT statements have a different number of columns

Here we get an error as the first SELECT statement returns three columns fname, lname, username while the second SELECT statement only returns two fname and lname.

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Published by Arthur Yanagisawa
Edited by 0 others
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