多表练习
-- SELECT c.*,t.* FROM course AS c,teacher as t WHERE c.cid=t.tid
-- SELECT gender,COUNT(sid) FROM student GROUP BY gender
-- SELECT s.*,b.* FROM score AS s,student as b WHERE s.sid=b.sid
-- SELECT * FROM teacher WHERE tname LIKE '李%'
-- SELECT s.*,c.* FROM student AS s,score AS c WHERE s.sid=c.sid
