1）承接上一节内容
mysql> show tables;
+------------------------+
| Tables_in_shoppingmall |
+------------------------+
| category |
| orderItem |
| orders |
| product |
| user |
+------------------------+
5 rows in set (0.01 sec)
2）多表查询内连接
-- 笛卡尔积： 查出来的是两张表的乘积，查出来没有实际意义
select * from product,category;
-- 过滤出有意义的数据
select * from product,category where cno = cid;
select * from product as p,category as c where p.cno = c.cid;
select * from product p,category c where p.cno = c.cid;
-- 内连接
隐式内连接
select * from product p,category c where p.cno = c.cid;
显式内连接
select * from product p inner join category c on p.cno = c.cid;
-- 区别：
隐式内连接：在查询出结果的基础上去做 where 条件过滤
显式内连接：带着条件去查询结果，只需的效率更高
- 数据如下
mysql> select * from product as p,category as c where p.cno = c.cid;
+-----+--------------+-------+------+-----+--------------+-----------------+
| pid | pname | price | cno | cid | cname | cdesc |
+-----+--------------+-------+------+-----+--------------+-----------------+
| 1 | iphone | 5300 | 1 | 1 | 电器 | 电器类商品 |
| 3 | 茅台 | 1800 | 2 | 2 | 副食品 | 烟酒类 |
| 9 | 五粮液 | 800 | 2 | 2 | 副食品 | 烟酒类 |
| 4 | 卫生纸 | 5 | 3 | 3 | 生活用品 | 用于生活 |
| 5 | 香蕉 | 10 | 4 | 4 | 水果 | 吃吃更健康 |
| 8 | 葡萄 | 15 | 4 | 4 | 水果 | 吃吃更健康 |
| 6 | 大白菜 | 30 | 5 | 5 | 蔬菜 | 绿色食品 |
| 7 | 三只松鼠 | 100 | 6 | 6 | 干果 | 好吃的零食 |
+-----+--------------+-------+------+-----+--------------+-----------------+
8 rows in set (0.00 sec)
3）左外、右外连接查询
-- 左外连接
左外连接，会将左表中的所有数据都查询出来，如果右表中没有对应的数据，则用null代替
准备工作：插入数据（cno = null）
insert into `product` values(null,'水杯',25,null);
select * from product p left outer join category c on p.cno = c.cid;
-- 右外连接
右外连接，会将右表中的所有数据查询出来，如果左表没有对应数据，则用null代替
准备工作：插入数据
insert into `category` values(100,'肉类','补充蛋白质');
select * from product p right outer join category c on p.cno = c.cid;
- 如下图
lianjie.png
- 左外连接数据如下
mysql> insert into `product` values(null,'水杯',25,null);
Query OK, 1 row affected (0.08 sec)
mysql> select * from product;
+-----+--------------+-------+------+
| pid | pname | price | cno |
+-----+--------------+-------+------+
| 1 | iphone | 5300 | 1 |
| 3 | 茅台 | 1800 | 2 |
| 4 | 卫生纸 | 5 | 3 |
| 5 | 香蕉 | 10 | 4 |
| 6 | 大白菜 | 30 | 5 |
| 7 | 三只松鼠 | 100 | 6 |
| 8 | 葡萄 | 15 | 4 |
| 9 | 五粮液 | 800 | 2 |
| 10 | 水杯 | 25 | NULL |
+-----+--------------+-------+------+
9 rows in set (0.00 sec)
mysql> select * from product p left outer join category c on p.cno = c.cid;
+-----+--------------+-------+------+------+--------------+-----------------+
| pid | pname | price | cno | cid | cname | cdesc |
+-----+--------------+-------+------+------+--------------+-----------------+
| 1 | iphone | 5300 | 1 | 1 | 电器 | 电器类商品 |
| 3 | 茅台 | 1800 | 2 | 2 | 副食品 | 烟酒类 |
| 4 | 卫生纸 | 5 | 3 | 3 | 生活用品 | 用于生活 |
| 5 | 香蕉 | 10 | 4 | 4 | 水果 | 吃吃更健康 |
| 6 | 大白菜 | 30 | 5 | 5 | 蔬菜 | 绿色食品 |
| 7 | 三只松鼠 | 100 | 6 | 6 | 干果 | 好吃的零食 |
| 8 | 葡萄 | 15 | 4 | 4 | 水果 | 吃吃更健康 |
| 9 | 五粮液 | 800 | 2 | 2 | 副食品 | 烟酒类 |
| 10 | 水杯 | 25 | NULL | NULL | NULL | NULL |
+-----+--------------+-------+------+------+--------------+-----------------+
9 rows in set (0.00 sec)
- 右外连接数据如下
mysql> select * from category;
+-----+--------------+-----------------+
| cid | cname | cdesc |
+-----+--------------+-----------------+
| 1 | 电器 | 电器类商品 |
| 2 | 副食品 | 烟酒类 |
| 3 | 生活用品 | 用于生活 |
| 4 | 水果 | 吃吃更健康 |
| 5 | 蔬菜 | 绿色食品 |
| 6 | 干果 | 好吃的零食 |
| 100 | 肉类 | 补充蛋白质 |
+-----+--------------+-----------------+
7 rows in set (0.00 sec)
mysql> select * from product p right outer join category c on p.cno = c.cid;
+------+--------------+-------+------+-----+--------------+-----------------+
| pid | pname | price | cno | cid | cname | cdesc |
+------+--------------+-------+------+-----+--------------+-----------------+
| 1 | iphone | 5300 | 1 | 1 | 电器 | 电器类商品 |
| 3 | 茅台 | 1800 | 2 | 2 | 副食品 | 烟酒类 |
| 9 | 五粮液 | 800 | 2 | 2 | 副食品 | 烟酒类 |
| 4 | 卫生纸 | 5 | 3 | 3 | 生活用品 | 用于生活 |
| 5 | 香蕉 | 10 | 4 | 4 | 水果 | 吃吃更健康 |
| 8 | 葡萄 | 15 | 4 | 4 | 水果 | 吃吃更健康 |
| 6 | 大白菜 | 30 | 5 | 5 | 蔬菜 | 绿色食品 |
| 7 | 三只松鼠 | 100 | 6 | 6 | 干果 | 好吃的零食 |
| NULL | NULL | NULL | NULL | 100 | 肉类 | 补充蛋白质 |
+------+--------------+-------+------+-----+--------------+-----------------+
9 rows in set (0.00 sec)
3）分页查询
分页查询 用到关键字 limit,第一个参数是索引，第二个参数是个数
select * from product limit 0,3;
select * from product limit 3,3;
- 分页查询数据如下
mysql> select * from product limit 0,3;
+-----+-----------+-------+------+
| pid | pname | price | cno |
+-----+-----------+-------+------+
| 1 | iphone | 5300 | 1 |
| 3 | 茅台 | 1800 | 2 |
| 4 | 卫生纸 | 5 | 3 |
+-----+-----------+-------+------+
3 rows in set (0.00 sec)
mysql> select * from product limit 3,3;
+-----+--------------+-------+------+
| pid | pname | price | cno |
+-----+--------------+-------+------+
| 5 | 香蕉 | 10 | 4 |
| 6 | 大白菜 | 30 | 5 |
| 7 | 三只松鼠 | 100 | 6 |
+-----+--------------+-------+------+
3 rows in set (0.00 sec)
3）子查询
00
-- 查询分类名称为“水果”的所有商品
1.查询分类名为“水果”的分类id
select cid from category where cname = '水果';
2.查询出id为4的结果
select * from product where cno = 4;
3.合并
select * from product where cno = (select cid from category where cname = '水果');
-- 查询出（商品名称、商品分类名称）信息
-- 左查询【查询出所有的信息了，不合适】
select * from product p left outer join category c on p.cno = c.cid;
-- 子查询
select pname,cno from product; //不合适
select pname,(select cname from category where product.cno = category.cid) from product; //不是很完美
select pname,(select cname from category where product.cno = category.cid) as 商品分类信息 from product; //尽管满足条件，但是不美观
//最优答案
select pname,(select cname from category as c where p.cno = c.cid) as 商品分类信息 from product as p;
//省略as
select pname,(select cname from category c where p.cno = c.cid) 商品分类信息 from product p;
- 子查询数据如下
`查询分类名称为“水果”的所有商品`
mysql> select cid from category where cname = '水果';
+-----+
| cid |
+-----+
| 4 |
+-----+
1 row in set (0.00 sec)
mysql> select * from product where con = 4;
ERROR 1054 (42S22): Unknown column 'con' in 'where clause'
mysql> select * from product where cno = 4;
+-----+--------+-------+------+
| pid | pname | price | cno |
+-----+--------+-------+------+
| 5 | 香蕉 | 10 | 4 |
| 8 | 葡萄 | 15 | 4 |
+-----+--------+-------+------+
2 rows in set (0.00 sec)
mysql> select * from product where cno = (select cid from category where cname = '水果');
+-----+--------+-------+------+
| pid | pname | price | cno |
+-----+--------+-------+------+
| 5 | 香蕉 | 10 | 4 |
| 8 | 葡萄 | 15 | 4 |
+-----+--------+-------+------+
2 rows in set (0.00 sec)
`查询出（商品名称、商品分类名称）信息`
mysql> select pname,(select cname from category c where p.cno = c.cid) 商品分类信息 from product p;
+--------------+--------------------+
| pname | 商品分类信息 |
+--------------+--------------------+
| iphone | 电器 |
| 茅台 | 副食品 |
| 卫生纸 | 生活用品 |
| 香蕉 | 水果 |
| 大白菜 | 蔬菜 |
| 三只松鼠 | 干果 |
| 葡萄 | 水果 |
| 五粮液 | 副食品 |
| 水杯 | NULL |
+--------------+--------------------+
9 rows in set (0.00 sec)
