What Are Aggregate Functions?

Aggregate Functions are functions where values of field expressions in each groups of selected rows are processed together as a single data set.

Groups of selected rows are normally defined by the GROUP BY clause. For example, COUNT() is an aggregate function that returns the number of elements of the input data set. For example:

SELECT help_category_id, COUNT(help_topic_id) FROM mysql.help_topic
  GROUP BY help_category_id;
  -- +------------------+----------------------+
  -- | help_category_id | count(help_topic_id) |
  -- +------------------+----------------------+
  -- |                1 |                    2 |
  -- |                2 |                   35 |
  -- |                3 |                   59 |
  -- |                4 |                    2 |
  -- |                5 |                    3 |
...
  -- +------------------+----------------------+

If the GROUP BY clause is not specified, an aggregate function will automatically assume all selected rows as a single group. For example:

SELECT COUNT(help_category_id), MIN(help_topic_id), MAX(help_topic_id)
  FROM mysql.help_topic;
  -- +-------------------------+--------------------+--------------------+
  -- | COUNT(help_category_id) | MIN(help_topic_id) | MAX(help_topic_id) |
  -- +-------------------------+--------------------+--------------------+
  -- |                     682 |                  0 |                681 |
  -- +-------------------------+--------------------+--------------------+

⇒ MySQL Functions on Character String Values

⇐ Argument Type Auto-Conversion

⇑ Calling MySQL Built-in Functions

⇑⇑ MySQL Function References