sql中的exists用法

查询选修语文（cno=21）的学生名单

SELECT sname  FROM student  WHERE  EXISTS ( SELECT  FROM  sc  WHERE sc.cno =  AND  sc.sno  =  student.sno )

查询没有选修语文（cno=21）的学生名单

SELECT sname  FROM student  WHERE NOT EXISTS ( SELECT  FROM  sc  WHERE sc.cno =  AND  sc.sno  =  student.sno )

选修全部课程的学生名单  （子查询）

SELECT sname  FROM student  WHERE sno  in  (

SELECT sc.sno  FROM  sc   GROUP BY  sc.sno  HAVING  count() =  (SELECT count() from course)

)

选修全部课程的学生名单 （exists）

select * from student t where not exists
( select * from course s where not exists
      (select * from sc where t.sno=sc.sno and s.cno= sc.cno   )
)

　当有课程没有选修时，下面的查询有返回的记录数。当全部课程都选择时，返回空

( select * from course s where not exists
      (select * from sc where t.sno=sc.sno and s.cno= sc.cno )
)

数据库表结构

--课程表
CREATE TABLE `course` (
  `cno` int() NOT NULL DEFAULT '',
  `cname` varchar() DEFAULT NULL,
  PRIMARY KEY (`cno`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8mb4;

INSERT INTO `course` VALUES ('', '数学');
INSERT INTO `course` VALUES ('', '语文');
INSERT INTO `course` VALUES ('', '外语');

--课程学生表
CREATE TABLE `sc` (
  `sno` int() NOT NULL DEFAULT '',
  `cno` int() NOT NULL DEFAULT '',
  PRIMARY KEY (`sno`,`cno`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8mb4;

INSERT INTO `sc` VALUES ('', '');
INSERT INTO `sc` VALUES ('', '');
INSERT INTO `sc` VALUES ('', '');
INSERT INTO `sc` VALUES ('', '');
INSERT INTO `sc` VALUES ('', '');
INSERT INTO `sc` VALUES ('', '');

--学生表
CREATE TABLE `student` (
  `sno` int() NOT NULL DEFAULT '',
  `sname` varchar() DEFAULT NULL,
  PRIMARY KEY (`sno`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8mb4;

INSERT INTO `student` VALUES ('', 'zhangsan');
INSERT INTO `student` VALUES ('', 'lisi');
INSERT INTO `student` VALUES ('', 'wangwu');