表连接查询

内连接
inner join

explain select count(*) from      BASE base  inner join A a on base.id = a.a_id  inner join B b on base.id = b.b_id 

结果等同于

explainselect  count(*) from     BASE base  ,A a  ,B bWHERE  base.id = a.project_id and  base.id = b.b_id 

需要满足每一个条件，数据量最小

左连接
left join

explain select count(*) from      BASE base  left join A a on base.id = a.a_id  left join B b on base.id = b.b_id 

以左边的表为主表进行数据组装，数据值跟

select count(*) from      BASE base  

相等

右连接

right join

explain select count(*) from      BASE base  right join A a on base.id = a.a_id  right join B b on base.id = b.b_id

以右边的表为主表进行数据组装