五個統計函數（單獨使用，意義不大，經常和分組group by組合使用）
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 所有值得行數有多少行
count（*）絕對行數null也計算在內
除此之外count（列名），計算這一列非null的行數
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 |
+-------------+
查詢類型爲4的庫存
select sum(goods_number) from goods where cat_id=4;
group by
統計一下每個類型分組下的庫存
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 |
+--------+-------------------+
不是標準的sql語句，邏輯上解釋不通（每個類別cat_id裏有很多goods_name）
不推薦 select goods_name ,sum(goods_number) from goods group by cat_id;
解釋：在select a/b中必須在group by a/b/c語意上纔沒有問題
技巧：查詢語句理解上從後面的條件過濾開始，先理解過濾條件，再看前面的執行