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Mysql重新整理笔记--woods备忘(四)
----+
| 1 | aaa | 1 |
| 2 | bbb | 2 |
| 3 | bbb | 4 |
+----+------+-------+
mysql> select * from tb;
+----+------+
| id | name |
+----+------+
| 1 | xxx |
| 2 | yyy |
| 3 | yyy |
+----+------+
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1.笛卡尔积查询：两张表中一条一条对应的记录，m条记录和n条记录查询，最后得到m*n条记录，其中很多错误数据
select * from ta ,tb; mysql> select * from ta ,tb; +----+------+-------+----+------+ | id | name | tb_id | id | name | +----+------+-------+----+------+ | 1 | aaa | 1 | 1 | xxx | | 2 | bbb | 2 | 1 | xxx | | 3 | bbb | 4 | 1 | xxx | | 1 | aaa | 1 | 2 | yyy | | 2 | bbb | 2 | 2 | yyy | | 3 | bbb | 4 | 2 | yyy | | 1 | aaa | 1 | 3 | yyy | | 2 | bbb | 2 | 3 | yyy | | 3 | bbb | 4 | 3 | yyy | +----+------+-------+----+------+
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2.内连接：查询两张表中都有的关联数据,相当于利用条件从笛卡尔积结果中筛选出了正确的结果。
select * from ta ,tb where ta.tb_id = tb.id; select * from ta inner join tb on ta.tb_id = tb.id; mysql> select * from ta inner join tb on ta.tb_id = tb.id; +----+------+-------+----+------+ | id | name | tb_id | id | name | +----+------+-------+----+------+ | 1 | aaa | 1 | 1 | xxx | | 2 | bbb | 2 | 2 | yyy | +----+------+-------+----+------+
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3.外连接
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（1）左外连接：在内连接的基础上增加左边有右边没有的结果
select * from ta left join tb on ta.tb_id = tb.id; mysql> select * from ta left join tb on ta.tb_id = tb.id; +----+------+-------+------+------+ | id | name | tb_id | id| name | +----+------+-------+------+------+ | 1 | aaa | 1 | 1 | xxx | | 2 | bbb | 2 | 2 | yyy | | 3 | bbb | 4 | NULL | NULL | +----+------+-------+------+------+
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（2）右外连接：在内连接的基础上增加右边有左边没有的结果
select * from ta right join tb on ta.tb_id = tb.id; mysql> select * from ta right join tb on ta.tb_id = tb.id; +------+------+-------+----+------+ | id| name | tb_id | id | name | +------+------+-------+----+------+ | 1 | aaa | 1 | 1 | xxx | | 2 | bbb | 2 | 2 | yyy | | NULL | NULL | NULL | 3 | yyy | +------+------+-------+----+------+
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（3）全外连接：在内连接的基础上增加左边有右边没有的和右边有左边没有的结果
select * from ta full join tb on ta.tb_id = tb.id; --mysql不支持全外连接 select * from ta left join tb on ta.tb_id = tb.id union select * from ta right join tb on ta.tb_id = tb.id; mysql> select * from ta left join tb on ta.tb_id = tb.id -> union -> select * from ta right join tb on ta.tb_id = tb.id; --mysql可以使用此种方式间接实现全外连接 +------+------+-------+------+------+ | id| name | tb_id | id| name | +------+------+-------+------+------+ | 1 | aaa | 1 | 1 | xxx | | 2 | bbb | 2 | 2 | yyy | | 3 | bbb | 4 | NULL | NULL | | NULL | NULL | NULL | 3 | yyy | +------+------+-------+------+------+
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