初始化数据
create database db_stu;
use db_stu;
create table stu(
student_id int primary key auto_increment,
student_name varchar(20),
student_class varchar(20)
);
insert into stu values(null, '张三', '0801');
insert into stu values(null, '李四', '0802');
insert into stu values(null, '王五', '0803');
insert into stu values(null, 'Tom', '0801');
create table sco(
id int primary key auto_increment,
score float,
student_id int,
object varchar(20)
);
insert into sco values(101, 80, 1, '语文');
insert into sco values(102, 90, 2, '语文');
insert into sco values(103, 88, 3, '语文');
| student_id | student_name | student_class |
|---|---|---|
| 1 | 张三 | 0801 |
| 2 | 李四 | 0802 |
| 3 | 王五 | 0803 |
| 4 | Tom | 0801 |
| id | score | student_id | object |
|---|---|---|---|
| 101 | 80 | 1 | 语文 |
| 102 | 90 | 2 | 语文 |
| 103 | 88 | 3 | 语文 |
内连接
普通内连接
- 关键字
inner join ... on - 内连接使用ON子句指定两个表的连接条件，WHERE子句来指定条件子句
mysql> select * from stu as a inner join sco as b;
+------------+--------------+---------------+-----+-------+------------+--------+
| student_id | student_name | student_class | id | score | student_id | object |
+------------+--------------+---------------+-----+-------+------------+--------+
| 1 | 张三 | 0801 | 101 | 80 | 1 | 语文 |
| 1 | 张三 | 0801 | 102 | 90 | 2 | 语文 |
| 1 | 张三 | 0801 | 103 | 88 | 3 | 语文 |
| 2 | 李四 | 0802 | 101 | 80 | 1 | 语文 |
| 2 | 李四 | 0802 | 102 | 90 | 2 | 语文 |
| 2 | 李四 | 0802 | 103 | 88 | 3 | 语文 |
| 3 | 王五 | 0803 | 101 | 80 | 1 | 语文 |
| 3 | 王五 | 0803 | 102 | 90 | 2 | 语文 |
| 3 | 王五 | 0803 | 103 | 88 | 3 | 语文 |
| 4 | Tom | 0801 | 101 | 80 | 1 | 语文 |
| 4 | Tom | 0801 | 102 | 90 | 2 | 语文 |
| 4 | Tom | 0801 | 103 | 88 | 3 | 语文 |
+------------+--------------+---------------+-----+-------+------------+--------+
12 rows in set (0.00 sec)
使用on语句关联两个表
mysql> select * from stu as a inner join sco as b on a.student_id = b.student_id;
+------------+--------------+---------------+-----+-------+------------+--------+
| student_id | student_name | student_class | id | score | student_id | object |
+------------+--------------+---------------+-----+-------+------------+--------+
| 1 | 张三 | 0801 | 101 | 80 | 1 | 语文 |
| 2 | 李四 | 0802 | 102 | 90 | 2 | 语文 |
| 3 | 王五 | 0803 | 103 | 88 | 3 | 语文 |
+------------+--------------+---------------+-----+-------+------------+--------+
3 rows in set (0.00 sec)
使用where条件
mysql> select * from stu as a inner join sco as b on a.student_id = b.student_id where a.student_name = '张三';
+------------+--------------+---------------+-----+-------+------------+--------+
| student_id | student_name | student_class | id | score | student_id | object |
+------------+--------------+---------------+-----+-------+------------+--------+
| 1 | 张三 | 0801 | 101 | 80 | 1 | 语文 |
+------------+--------------+---------------+-----+-------+------------+--------+
1 row in set (0.00 sec)
隐式内连接（用的是最多的）
- 可以不使用inner join ... on关键字
mysql> select * from stu,sco where stu.student_id = sco.student_id;
+------------+--------------+---------------+-----+-------+------------+--------+
| student_id | student_name | student_class | id | score | student_id | object |
+------------+--------------+---------------+-----+-------+------------+--------+
| 1 | 张三 | 0801 | 101 | 80 | 1 | 语文 |
| 2 | 李四 | 0802 | 102 | 90 | 2 | 语文 |
| 3 | 王五 | 0803 | 103 | 88 | 3 | 语文 |
+------------+--------------+---------------+-----+-------+------------+--------+
3 rows in set (0.00 sec)
外连接
左外连接(看左表，把左表所有的数据全部查询出来)
- 关键字
left [outer] join ... on, 可以不用写outer关键字
mysql> select * from stu as a left join sco as b on a.student_id = b.student_id;
+------------+--------------+---------------+------+-------+------------+--------+
| student_id | student_name | student_class | id | score | student_id | object |
+------------+--------------+---------------+------+-------+------------+--------+
| 1 | 张三 | 0801 | 101 | 80 | 1 | 语文 |
| 2 | 李四 | 0802 | 102 | 90 | 2 | 语文 |
| 3 | 王五 | 0803 | 103 | 88 | 3 | 语文 |
| 4 | Tom | 0801 | NULL | NULL | NULL | NULL |
+------------+--------------+---------------+------+-------+------------+--------+
4 rows in set (0.00 sec)
右外连接(看右表，把右表所有的数据全部查询出来)
- 使用关键字 right [outer] join ... on
mysql> select * from stu as a right join sco as b on a.student_id = b.student_id;
+------------+--------------+---------------+-----+-------+------------+--------+
| student_id | student_name | student_class | id | score | student_id | object |
+------------+--------------+---------------+-----+-------+------------+--------+
| 1 | 张三 | 0801 | 101 | 80 | 1 | 语文 |
| 2 | 李四 | 0802 | 102 | 90 | 2 | 语文 |
| 3 | 王五 | 0803 | 103 | 88 | 3 | 语文 |
+------------+--------------+---------------+-----+-------+------------+--------+
3 rows in set (0.00 sec)
完全连接
MySQL不支持完全连接
交叉连接
- 不带条件的话会返回两个表的笛卡尔积
- 如果带on或者where语句之后, 会先生成两个表行数乘积的数据表，然后才根据条件从中选择
mysql> select * from stu as a cross join sco as b;
+------------+--------------+---------------+-----+-------+------------+--------+
| student_id | student_name | student_class | id | score | student_id | object |
+------------+--------------+---------------+-----+-------+------------+--------+
| 1 | 张三 | 0801 | 101 | 80 | 1 | 语文 |
| 1 | 张三 | 0801 | 102 | 90 | 2 | 语文 |
| 1 | 张三 | 0801 | 103 | 88 | 3 | 语文 |
| 2 | 李四 | 0802 | 101 | 80 | 1 | 语文 |
| 2 | 李四 | 0802 | 102 | 90 | 2 | 语文 |
| 2 | 李四 | 0802 | 103 | 88 | 3 | 语文 |
| 3 | 王五 | 0803 | 101 | 80 | 1 | 语文 |
| 3 | 王五 | 0803 | 102 | 90 | 2 | 语文 |
| 3 | 王五 | 0803 | 103 | 88 | 3 | 语文 |
| 4 | Tom | 0801 | 101 | 80 | 1 | 语文 |
| 4 | Tom | 0801 | 102 | 90 | 2 | 语文 |
| 4 | Tom | 0801 | 103 | 88 | 3 | 语文 |
+------------+--------------+---------------+-----+-------+------------+--------+
12 rows in set (0.00 sec)
mysql> select * from stu as a right join sco as b on a.student_id = b.student_id;
+------------+--------------+---------------+-----+-------+------------+--------+
| student_id | student_name | student_class | id | score | student_id | object |
+------------+--------------+---------------+-----+-------+------------+--------+
| 1 | 张三 | 0801 | 101 | 80 | 1 | 语文 |
| 2 | 李四 | 0802 | 102 | 90 | 2 | 语文 |
| 3 | 王五 | 0803 | 103 | 88 | 3 | 语文 |
+------------+--------------+---------------+-----+-------+------------+--------+
3 rows in set (0.00 sec)
总结一下:
inner join ... on ... where ...
left join ... on ... where ...
right join ... on ... where ...
cross join ... on ...
