SELECT * FROM A WHERE id IN (SELECT id FROM B); 等价于：1、SELECT id FROM B ----->先执行in中的查询 2、SELECT * FROM A WHERE A.id = B.id 以上in()中的查询只执行一次，它查询出B中的所有的id并缓存起来，然后检查A表中查询出的id在缓存中是否存在，如果存在则将A的查询数据加入到结果 ...

This should be a quickie.<BR><BR>I need to write a query that has to run in a tight loop in my application, so I want to make it efficient.<BR><BR>Is there any significant difference in efficiency ...