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MySQL | COUNT method

schedule Aug 10, 2023
Last updated
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MySQL
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MySQL's COUNT(~) aggregate method counts the number of returned results.

Parameters

1. expr | expression

The expression to count the number of retrieved rows for.

Return value

The number of rows in expr that do not contain NULL values.

WARNING

The exception is for COUNT(*) which counts retrieved rows including those containing NULL values.

Examples

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 count the number of clubs in extracurricular table:

SELECT COUNT(club)
FROM extracurricular;
+-------------+
| COUNT(club) |
+-------------+
| 4 |
+-------------+

Note that the NULL value for student_id=5 is ignored, which is why we have a return value of 4.

COUNT(*)

To return the number of records in the extracurricular table:

SELECT COUNT(*)
FROM extracurricular;
+----------+
| COUNT(*) |
+----------+
| 5 |
+----------+

Note that COUNT(*) returns a count of the number of rows retrieved, whether or not they contain NULL values.

Distinct records

To count the number of distinct clubs in extracurricular table:

SELECT COUNT(DISTINCT club)
FROM extracurricular;
+----------------------+
| COUNT(DISTINCT club) |
+----------------------+
| 4 |
+----------------------+

Missing values

To count missing values, use the IS NULL operator like so:

SELECT COUNT(*)
FROM extracurricular
WHERE club IS NULL;
+----------+
| COUNT(*) |
+----------+
| 1 |
+----------+

We can see that there is one row in table extracurricular with a NULL club value.

Note that the following SQL does not work:

SELECT COUNT(*)
FROM extracurricular
WHERE club = NULL;
robocat
Published by Arthur Yanagisawa
Edited by 0 others
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