什么是mysql中的聚合列？

Question

What are aggregated columns and non-aggregated columns? I have read the MySQL Handling of GROUP BY and am confused. I do not know what 'aggregated columns' mean. Does anybody know?

MySQL Handling of GROUP BY link

Answer 1

Functions like SUM , AVG , MAX , MIN , COUNT calculate data over a group of records and return aggregated results.

Eg

SELECT SUM(`salary`) FROM `employees`;

Returns 1 row with the total salary when

SELECT `salary` FROM `employees`;

Returns multiple rows with the salary per employee.

Lets say you want the average salary per gender:

SELECT `gender`, AVG(`salary`) FROM `employees` GROUP BY `gender`;

Query 1 & 3 contain aggregated columns: SUM(`salary`) AND AVG(`salary`)

Answer 2

SUM , AVG , COUNT , MIN , MAX etc all are aggregate functions which create a column as a result of combining multiple rows.

Aggregated columns are columns returned as a result of applying aggregate function ( SUM , AVG , COUNT , MIN , MAX )