EXISTS 指定一个子查询，检测行的存在。
SELECT DISTINCT name FROM student WHERE EXISTS (SELECT * FROM register WHERE id = student.id);
(note that if some rows in student register.id =student.id, all those rows in student are selected out.)
更多请看下节：http://www.mark-to-win.com/tutorial/mydb_DBIntroduction_ExistOper.html
