SQL 中 EXISTS 与 NOT EXISTS

带有 EXISTS 操作符的子查询不返回任何数据，只产生逻辑真值 ‘true‘ 或逻辑假值 ‘false‘。带有 EXISTS 操作符的子查询都是相关子查询。

相关子查询：子查询的条件依赖父查询。

EXISTS：如果内层查询结果非空，则外层 WHERE 子句返回真值，输出外层查询结果。

NOT EXISTS：如果内层查询结果为空，则外层 WHERE 子句返回真值，输出外层查询结果。

样例表

CREATE TABLE Student
(Sno        CHAR(9) PRIMARY KEY,        --学号
 Sname      CHAR(20) UNIQUE,            --姓名
);
create table SC
(sno        char(9),                    --学号
 cno        char(4),                    --课程号
 primary key (sno,cno),
 foreign key (sno) references student(sno)
);

/*查询至少有一门课没选的学生姓名*/
SELECT sname
FROM Student
WHERE EXISTS (SELECT *
              FROM Course
              WHERE NOT EXISTS(SELECT *
                               FROM SC
                               WHERE Sno=Student.Sno AND Cno=Course.Cno));
-- 外层要有查询结果     中层查询结果要为非空         内层查询结果要为空      =  至少有一门课没选

/*查询所有课都没选的学生姓名*/
SELECT sname
FROM Student
WHERE NOT EXISTS (SELECT *
                  FROM Course
                  WHERE  EXISTS (SELECT *
                                 FROM SC
                                 WHERE Sno=Student.Sno AND Cno=Course.Cno));
--外层要有查询结果      中层查询结果要为空            内层查询结果要为空    =    所有的课都没选

/*查询至少选了一门课的学生姓名*/
SELECT sname
FROM Student
WHERE  EXISTS (SELECT *
               FROM Course
               WHERE  EXISTS (SELECT *
                              FROM SC
                              WHERE Sno=Student.Sno AND Cno=Course.Cno));
--外层要有查询结果          中层查询结果要为非空      内层查询结果要为非空   =    选了至少一门课

/*查询选了所有课的学生姓名*/
SELECT sname
FROM Student
WHERE NOT EXISTS (SELECT *
                  FROM Course
                  WHERE NOT EXISTS(SELECT *
                                   FROM SC
                                   WHERE Sno=Student.Sno AND Cno=Course.Cno));
--外层要有查询结果           中层查询结果要为空        内层查询结果要为非空    =    选了所有的课

/*用 GROUP BY */
SELECT sname
FROM Student LEFT JOIN SC ON Student.Sno=SC.sno
GROUP BY Sname
HAVING COUNT(cno)<= (SELECT COUNT(cno)
                     FROM Course)-1;

SELECT sname
FROM Student LEFT JOIN SC ON Student.Sno=SC.sno
GROUP BY Sname
HAVING COUNT(cno)= 0;

SELECT sname
FROM Student LEFT JOIN SC ON Student.Sno=SC.sno
GROUP BY Sname
HAVING COUNT(cno)>= 1;

SELECT sname
FROM Student LEFT JOIN SC ON Student.Sno=SC.sno
GROUP BY Sname
HAVING COUNT(cno)= (SELECT COUNT(cno)
                    FROM Course);

/*查询至少选修了学生 201215122 选修的全部课程*/
SELECT DISTINCT sno
FROM SC x
WHERE NOT EXISTS (SELECT *
                  FROM SC Y
                  WHERE Y.sno=201215122 AND NOT EXISTS(SELECT *
                                                       FROM SC Z
                                                       WHERE Z.sno=X.sno AND Z.cno=Y.cno));
--外层要有查询结果     中层查询结果要为空                内层查询结果要为非空 = 至少选修了201215122 选修的全部课程 

SELECT cno
INTO copy
FROM SC
WHERE sno=201215122;   --复制 201215122 选的课号到 copy 表

SELECT DISTINCT sno
FROM copy LEFT OUTER JOIN SC ON SC.cno=copy.cno  --以 copy 表为全部行
GROUP BY sno
HAVING COUNT(SC.cno)= (SELECT COUNT(cno)   --排序只选了 copy 表中的一门或几门课的 sno
                       FROM copy);