关于 JOIN 语句条件的筛选优化问题
select a.xx,b.xx from a left join b on a.id = b.id where a.id = 1
select a.xx,b.xx from a left join b on a.id = b.id and a.id = 1
select a1.xx,b.xx from (select a.xx from a where a.id = 1) a1 left join b on a1.id = b.id
请问以上三个语句哪个效率高?