左联后再内联的2种写法

select t1.user_id
from a t1
left join
b t2
on
t1.utm = t2.utm     #先执行左链接
join c t3 on t2.id = t3.id;  #然后再把左联的表和c表联查询

 上下2种写法都是一样的

select * from (select t1.user_id from a t1 left join b t2 on t1.utm = t2.utm) as lucax

join c t3 on lucax.user_id = t3.id