五个统计函数（单独使用，意义不大，经常和分组group by组合使用）
max 最大 select max(shop_price) from goods;
min 最小 select min(shop_price) from goods;
sum 求和 select sum(shop_price) from goods;
avg 求平均值 select avg(shop_price) from goods;
count 所有值得行数有多少行
count（*）绝对行数null也计算在内
除此之外count（列名），计算这一列非null的行数
count使用
mysql> select * from test8;
+------+------+
| id | name |
+------+------+
| 1 | lisi |
| 2 | NULL |
+------+------+
mysql> select count(*) from test8;
+----------+
| count(*) |
+----------+
| 2 |
+----------+
mysql> select count(name) from test8;
+-------------+
| count(name) |
+-------------+
| 1 |
+-------------+
查询类型为4的库存
select sum(goods_number) from goods where cat_id=4;
group by
统计一下每个类型分组下的库存
mysql> select cat_id,sum(goods_number) from goods group by cat_id;
+--------+-------------------+
| cat_id | sum(goods_number) |
+--------+-------------------+
| 2 | 0 |
| 3 | 203 |
| 4 | 4 |
| 5 | 8 |
| 8 | 61 |
| 11 | 23 |
| 13 | 4 |
| 14 | 9 |
| 15 | 2 |
+--------+-------------------+
不是标准的sql语句，逻辑上解释不通（每个类别cat_id里有很多goods_name）
不推荐 select goods_name ,sum(goods_number) from goods group by cat_id;
解释：在select a/b中必须在group by a/b/c语意上才没有问题
技巧：查询语句理解上从后面的条件过滤开始，先理解过滤条件，再看前面的执行