那七个东西简单模糊,明日来讲讲
一、准备工作
率先创制 2 个表 t1 和 t2, 并插入数据
CREATE TABLE `t1` (
`id1` int(11) NOT NULL AUTO_INCREMENT,
`num1` int(11) DEFAULT NULL,
PRIMARY KEY (`id1`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=latin1
insert into t1 values(1,10),(2,20),(3,30),(4,40),(5,50);
table t1.png
CREATE TABLE `t2` (
`id2` int(11) NOT NULL AUTO_INCREMENT,
`num2` int(11) DEFAULT NULL,
PRIMARY KEY (`id2`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=latin1
insert into t2 values(1,100),(3,300),(5,500),(7,700),(9,900);
table t2.png
二、left join
官方解释,装逼用的,可以跳过
MySQL implements an A LEFT JOIN B join_condition as follows:
- Table B is set to depend on table A and all tables on which A
depends. - Table A is set to depend on all tables (except B) that are used in
the LEFT JOIN condition. - If there is a row in A that matches the WHERE clause, but there is
no row in B that matches the ON condition, an extra B row is
generated with all columns set to NULL.
在极端输入select * from t1 left join t2 on t1.id1=t2.id2;
出口如下
left join.png
也就是说,表t1左联接表t2, 左侧t1是老大,左侧t2是维护者
表t1负有的笔录都会显得出来,
而表t2只会浮现出满意join_condition的口径的记录,即ti.id1=t2.id2,如若存在t1有的记录而t2没有,则体现NULL
三、right join
MySQL,right join只是刚刚反过来而已,左边的表是老大,左侧的是维护者
在巅峰输入select * from t1 right join t2 on t1.id1=t2.id2;
输出如下
right join.png
详情请看官方解释