假设student表有如下记录： +--------+----------+---------+-----------+| stu_id | stu_name | stu_tel | stu_score |+--------+----------+---------+-----------+| 1 | a | 151 | 60 || 2 | b | 152 | 61 || 3 | c | 153 | 62 |+--------+----------+---
假设student表有如下记录：
+--------+----------+---------+-----------+ | stu_id | stu_name | stu_tel | stu_score | +--------+----------+---------+-----------+ | 1 | a | 151 | 60 | | 2 | b | 152 | 61 | | 3 | c | 153 | 62 | +--------+----------+---------+-----------+
teacher表有如下记录：
+--------+----------+---------+---------+--------+ | tea_id | tea_name | tea_tel | tea_pay | stu_id | +--------+----------+---------+---------+--------+ | 1 | A | 130 | 3000 | 1 | | 2 | B | 131 | 4000 | 2 | | 3 | C | 132 | 4000 | 0 | +--------+----------+---------+---------+--------+
连接可分为以下几类：
内连接。
内连接使用比较运算符根据每个表共有的列的值匹配两个表中的行,也可以使用显示内连接：inner join ，没有包含一个连接条件时，;结果就是一个笛卡尔积。
mysql> select * from student inner join teacher;（类似于select * from student,teacher;） +--------+----------+---------+-----------+--------+----------+---------+---------+--------+ | stu_id | stu_name | stu_tel | stu_score | tea_id | tea_name | tea_tel | tea_pay | stu_id | +--------+----------+---------+-----------+--------+----------+---------+---------+--------+ | 1 | a | 151 | 60 | 1 | A | 130 | 3000 | 1 | | 2 | b | 152 | 61 | 1 | A | 130 | 3000 | 1 | | 3 | c | 153 | 62 | 1 | A | 130 | 3000 | 1 | | 1 | a | 151 | 60 | 2 | B | 131 | 4000 | 2 | | 2 | b | 152 | 61 | 2 | B | 131 | 4000 | 2 | | 3 | c | 153 | 62 | 2 | B | 131 | 4000 | 2 | | 1 | a | 151 | 60 | 3 | C | 132 | 4000 | 0 | | 2 | b | 152 | 61 | 3 | C | 132 | 4000 | 0 | | 3 | c | 153 | 62 | 3 | C | 132 | 4000 | 0 | +--------+----------+---------+-----------+--------+----------+---------+---------+--------+
外连接。外连接可以是左向外连接、右向外连接。
在FROM子句中指定外连接时，可以由下列几组关键字中的一组指定：
左向外连接：left join 或者left outer join
左向外连接的结果集包括left join子句中指定的左表的所有行，而不仅仅是连接列所匹配的行。如果左表的某行在右表中没有匹配行，则在相关联的结果集行中右表的所有选择列表列均为空值。
mysql> select * from student left join teacher on student.stu_id=teacher.stu_id; +--------+----------+---------+-----------+--------+----------+---------+---------+--------+ | stu_id | stu_name | stu_tel | stu_score | tea_id | tea_name | tea_tel | tea_pay | stu_id | +--------+----------+---------+-----------+--------+----------+---------+---------+--------+ | 1 | a | 151 | 60 | 1 | A | 130 | 3000 | 1 | | 2 | b | 152 | 61 | 2 | B | 131 | 4000 | 2 | | 3 | c | 153 | 62 | NULL | NULL | NULL | NULL | NULL | +--------+----------+---------+-----------+--------+----------+---------+---------+--------+
右向外连接：right join 或 right outer join 。
右向外连接是左向外连接的反向连接。将返回右表的所有行。如果右表的某行在左表中没有匹配行，则将为左表返回空值。
mysql> select * from student right join teacher on student.stu_id=teacher.stu_id; +--------+----------+---------+-----------+--------+----------+---------+---------+--------+ | stu_id | stu_name | stu_tel | stu_score | tea_id | tea_name | tea_tel | tea_pay | stu_id | +--------+----------+---------+-----------+--------+----------+---------+---------+--------+ | 1 | a | 151 | 60 | 1 | A | 130 | 3000 | 1 | | 2 | b | 152 | 61 | 2 | B | 131 | 4000 | 2 | | NULL | NULL | NULL | NULL | 3 | C | 132 | 4000 | 0 | +--------+----------+---------+-----------+--------+----------+---------+---------+--------+
自然连接
在连接条件中使用等于(=)运算符比较被连接列的列值，但它使用选择列表指出查询结果集合中所包括的列，并删除连接表中的重复列。
mysql> select * from student natural join teacher; +--------+----------+---------+-----------+--------+----------+---------+---------+ | stu_id | stu_name | stu_tel | stu_score | tea_id | tea_name | tea_tel | tea_pay | +--------+----------+---------+-----------+--------+----------+---------+---------+ | 1 | a | 151 | 60 | 1 | A | 130 | 3000 | | 2 | b | 152 | 61 | 2 | B | 131 | 4000 | +--------+----------+---------+-----------+--------+----------+---------+---------+
交叉连接。交叉连接返回左表中的所有行，左表中的每一行与右表中的所有行组合。如果没有包含一个连接条件，交叉连接的结果就是一个笛卡尔积。
mysql> select * from student cross join teacher; +--------+----------+---------+-----------+--------+----------+---------+---------+--------+ | stu_id | stu_name | stu_tel | stu_score | tea_id | tea_name | tea_tel | tea_pay | stu_id | +--------+----------+---------+-----------+--------+----------+---------+---------+--------+ | 1 | a | 151 | 60 | 1 | A | 130 | 3000 | 1 | | 2 | b | 152 | 61 | 1 | A | 130 | 3000 | 1 | | 3 | c | 153 | 62 | 1 | A | 130 | 3000 | 1 | | 1 | a | 151 | 60 | 2 | B | 131 | 4000 | 2 | | 2 | b | 152 | 61 | 2 | B | 131 | 4000 | 2 | | 3 | c | 153 | 62 | 2 | B | 131 | 4000 | 2 | | 1 | a | 151 | 60 | 3 | C | 132 | 4000 | 0 | | 2 | b | 152 | 61 | 3 | C | 132 | 4000 | 0 | | 3 | c | 153 | 62 | 3 | C | 132 | 4000 | 0 | +--------+----------+---------+-----------+--------+----------+---------+---------+--------+
如果要连接的列的名字相同，并且连接条件就是两者相等，那么也可以使用using，只需把on语句用using替换即可；
mysql> select * from student inner join teacher using(stu_id); +--------+----------+---------+-----------+--------+----------+---------+---------+ | stu_id | stu_name | stu_tel | stu_score | tea_id | tea_name | tea_tel | tea_pay | +--------+----------+---------+-----------+--------+----------+---------+---------+ | 1 | a | 151 | 60 | 1 | A | 130 | 3000 | | 2 | b | 152 | 61 | 2 | B | 131 | 4000 | +--------+----------+---------+-----------+--------+----------+---------+---------+
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