http://dev.mysql.com/doc/refman/5.0/en/exists-and-not-exists-subqueries.html
SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);
t2가 존재한다면 t1을 SELECT 하게 된다..
http://dev.mysql.com/doc/refman/5.0/en/exists-and-not-exists-subqueries.html
SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);
t2가 존재한다면 t1을 SELECT 하게 된다..
