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create table a(userid int,name varchar(20),age int);
create table b(id int,userid int)
select a.userid,a.name,a.age
from A a
inner loop join B b
on a.userid = b.userid and b.id = '';
select b.userid,
(select a.name from A a where a.userid = b.userid) as name,
(select a.age from A a where a.userid = b.userid) as age
from B b
where b.id = '';
从执行计划来看,第一种优于第二种,而且执行计划非常简洁,而第二种方式的执行计划复杂的多。