sqlserver exists463.com,not exists的用法

学生表:create table student(id number(8) primary key,name varchar2(10),deptment number(8))

选课表:create table select_course( ID NUMBER(8) primary key, STUDENT_ID NUMBER(8) foreign key (COURSE_ID) references course(ID), COURSE_ID NUMBER(8) foreign key (STUDENT_ID) references student(ID))

课程表:create table COURSE( ID NUMBER(8) not null, C_NAME VARCHAR2(20), C_NO VARCHAR2(10))

student表的数据: ID NAME DEPTMENT_ID---------- --------------- ----------- 1 echo 1000 2 spring 2000 3 smith 1000 4 liter 2000

course表的数据: ID C_NAME C_NO---------- -------------------- -------- 1 数据库 data1 2 数学 month1 3 英语 english1

select_course表的数据: ID STUDENT_ID COURSE_ID---------- ---------- ---------- 1 1 1 2 1 2 3 1 3 4 2 1 5 2 2 6 3 2

1.查询选修了所有课程的学生id、name:

分析:如果有一门课没有选,则此时(1)select * from select_course sc where sc.student_id=ts.id 

and sc.course_id=c.id存在null,

这说明(2)select * from course c 的查询结果中确实有记录不存在(1查询中),查询结果返回没有选的课程,

此时select * from t_student ts 后的not exists 判断结果为false,不执行查询。

SQL select * from t_student ts where not exists(select * from course c where not exists(select * from select_course sc where sc.student_id=ts.id and sc.course_id=c.id)); 

 ID NAME DEPTMENT_ID---------- --------------- ----------- 1 echo 1000

2.查询没有选择所有课程的学生,即没有全选的学生。,

分析:只要有一个门没有选,即select * from select_course sc where student_id=t_student.id and course_id=course.id 有一条为空,即not exists null 为true,此时select * from course有查询结果,

因此select id,name from t_student 将执行查询。

SQL select id,name from t_student where exists

(select * from course where not exists

(select * from select_course sc where student_id=t_student.id and course_id=course.id));

 ID NAME---------- --------------- 2 spring 3 smith 4 liter

3.查询一门课也没有选的学生。,

分析:如果他选修了一门select * from course结果集不为空,not exists 判断结果为false;

select id,name from t_student 不执行查询。

SQL select id,name from t_student where not exists

(select * from course where exists

(select * from select_course sc where student_id=t_student.id and course_id=course.id));

 ID NAME---------- --------------- 4 liter

4.查询至少选修了一门课程的学生。SQL select id,name from t_student where exists

(select * from course where exists

(select * from select_course sc where student_id=t_student.id and course_id=course.id));

 ID NAME---------- --------------- 1 echo 2 spring 3 smith

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