mysql 的 join
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基本格式:
select ... from A
join B1 on ...
join B2 on ...
...
where ...
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join 类型
join 内连接
left join 左连接,左表所有数据查出,右表跟左表有关联的数据才查出
right join 右连接,右表所有数据查出,左表跟右表有关联的数据才查出
inner join 内连接,同 join,
全连接 mysql 不支持全连接,可以通过 union 2个 left join 或 right join 实现全连接,
= 如果不用 join 语句,直接用 = 号关联,则相当于 内连接,
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例子:
* 建表:
-- create table boy & gril create table boy( id int(10) primary key auto_increment, name varchar(50) not null, girl_id int null, cdate timestamp default current_timestamp ) engine=innodb default charset=utf8; create table girl( id int(10) primary key auto_increment, name varchar(50) not null, boy_id int null, cdate timestamp default current_timestamp ) engine=innodb default charset=utf8;
* 插入测试数据:
-- clear table truncate table boy; truncate table girl; -- init data insert into boy(name,girl_id) values ('boy_01',2), ('boy_02',5), ('boy_03',1), ('boy_04',4), ('boy_05',3), ('boy_06',100); insert into girl(name,boy_id) values ('girl_1',3), ('girl_2',1), ('girl_3',5), ('girl_4',4), ('girl_5',2), ('girl_6',100);
* 各种 join 的 sql 语句:
-- 内连接 select b.id as boy_id, g.id as girl_id from boy b join girl g on b.girl_id = g.id order by b.id asc; -- 用 = 关联,相当于 内连接 select b.id as boy_id, g.id as girl_id from boy b, girl g where b.girl_id = g.id order by b.id asc; -- 左连接 select b.id as boy_id, g.id as girl_id from boy b left join girl g on b.girl_id = g.id order by b.id asc; -- 右连接 select b.id as boy_id, g.id as girl_id from boy b right join girl g on b.girl_id = g.id order by b.id asc; -- 2个 左连接,通过 union 合并,相当于 全连接 select b.id as boy_id, g.id as girl_id from boy b left join girl g on b.girl_id = g.id union select b.id as boy_id, g.id as girl_id from girl g left join boy b on g.boy_id = b.id
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