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MySQL | HAVING

Database
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MySQL
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Documentation
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Statements and Clauses
schedule Jul 1, 2022
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MySQL's HAVING clause is used to specify a condition involving aggregate functions. It is used in GROUP BY queries, and specifies a condition upon the grouped results.

NOTE

The WHERE clause specifies a condition that a record must meet before it is considered for grouping, while the HAVING clause specifies the condition that should be applied after the grouping and aggregation takes place.

Syntax

SELECT column_name(s)
FROM table_name
GROUP BY column
HAVING condition
ORDER BY column;

Examples

Consider the following table about whether students hold a drivers license:

student_id

full_name

age

has_license

1

Sky Towner

17

0

2

Ben Davis

19

1

3

Travis Apple

18

0

4

Arthur David

16

0

5

Benjamin Town

17

1

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

Basic usage

To find the ages where there is only one student of that age:

SELECT age, COUNT(*)
FROM drivers_license
GROUP BY age
HAVING COUNT(*) = 1;
+------+----------+
| age | COUNT(*) |
+------+----------+
| 19 | 1 |
| 18 | 1 |
| 16 | 1 |
+------+----------+

Note age 17 is not returned as there are two students who are 17.

Using HAVING with WHERE

To include a WHERE clause to filter out students with full_name='Sky Towner' before we perform grouping:

SELECT age, COUNT(*)
FROM drivers_license
WHERE full_name <> 'Sky Towner'
GROUP BY age
HAVING COUNT(*) = 1;
+------+----------+
| age | COUNT(*) |
+------+----------+
| 19 | 1 |
| 18 | 1 |
| 16 | 1 |
| 17 | 1 |
+------+----------+

This time we have 4 records returned as 'Sky Towner' is filtered out by the WHERE clause before we perform grouping. Therefore, of the records that were aggregated / grouped, 'Benjamin Town' was the only 17 year old, hence the HAVING clause was fulfilled and age 17 was returned.

robocat
Published by Arthur Yanagisawa
Edited by 0 others
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