笛卡尔积
SELECT * FROM stu ,class;
SELECT * FROM stu ,class WHERE stu.class_id = class.id;INNER JOIN
SELECT * FROM stu AS s INNER JOIN user_info as i ON s.id = i.stu_id;
SELECT * FROM stu INNER JOIN class ON stu.class_id = class.id;
SELECT sname,class_id,stu.id as stu_id,sex,cname FROM stu INNER JOIN class ON stu.class_id = class.id WHERE class.id = 1;