这2个SQL的效率有区别吗

Question

这2种SQL执行起来似乎感觉不到区别？

select * from A 
left join B on A.aid=B.aid
where A.aid=100

select * from (
    select * from A where A.aid=100
) AA
left join B on AA.aid=B.aid

怎么看SQL执行解释分析呢？

Answer 1

EXPLAIN select * from A 
left join B on A.aid=B.aid
where A.aid=100;

EXPLAIN select * from (
    select * from A where A.aid=100
) AA
left join B on AA.aid=B.aid

看看两个语句的执行计划。