- 聚集函数：对（一列的）某些行运行计算的函数，计算并返回一个值。
| 函数 | 说明 |
|---|---|
| AVG() | 返回平均值 |
| COUNT() | 行数 |
| MAX() | 最大值 |
| MIN() | 最小值 |
| SUM() | 最小值 |
mysql> select avg(prod_price) as avg_price from Products;
+-----------+
| avg_price |
+-----------+
| 6.823333 |
+-----------+
1 row in set (0.00 sec)
mysql> select avg(prod_price) as avg_price from Products where vend_id = 'DLL01';
+-----------+
| avg_price |
+-----------+
| 3.865000 |
+-----------+
1 row in set (0.00 sec)
只计算不同的值
mysql> select avg( distinct prod_price) as avg_price from Products where vend_id = 'DLL01';
+-----------+
| avg_price |
+-----------+
| 4.240000 |
+-----------+
1 row in set (0.00 sec)
组合集聚函数
mysql> select count(*) as num_items,
-> min(prod_price) as price_min,
-> max(prod_price) as price_max,
-> avg(prod_price) as price_avg
-> from Products;
+-----------+-----------+-----------+-----------+
| num_items | price_min | price_max | price_avg |
+-----------+-----------+-----------+-----------+
| 9 | 3.49 | 11.99 | 6.823333 |
+-----------+-----------+-----------+-----------+
1 row in set (0.00 sec)
