Grouping and Aggregate Functions
Grouping Concepts
HAVING
- because the
GROUP BY must be after the WHERE
Aggregate Functions
| function |
description |
max() |
返回最大值 |
min() |
返回最小值 |
avg() |
返回均值 |
sum() |
返回总和 |
count() |
计数 |
Implicit V.S. Explicit Groups
- Implicit group: 不分组，直接使用max(),min()等aggregate function.(相当将所有的数据分到同一组)
- Explicit group: 使用
GROUP BY进行分组
Counting Distinct Values
SELECT COUNT(DISTINCT $fld_name) FROM $tb_name;
Using Expressions
SELECT MAX(datediff($fld_name1,$fld_name2))
Null
- ignore
null
count($fld_name) sum($fld_name) avg($fld_name)
- not ignore
null