http://www.cnblogs.com/glory-jzx/archive/2012/07/19/2599215.html
http://sunxiaqw.blog.163.com/blog/static/990654382013430105130443/
这篇文章写的很清楚了，请参考,多谢原作者。
select loan_user_id from sp_invest_project p where exists(
select 1 from temp_stock_enterprise e where e.user_id = p.loan_user_id
) ;
select loan_user_id from sp_invest_project p where loan_user_id in (
select user_id from temp_stock_enterprise
) ;
上述两种写法应该是一致的。
原文：http://www.cnblogs.com/daixianjun/p/mysql-exists.html