Question

我有三个相关的表（学生，班级，注册）（一个学生可以有多个注册，一个班可以有多个注册），我需要从中选择所有注册课程的学生（x，y，z）但不在班级（a，b，c）。如何构建我的查询来实现这一目标？

Answer 1

我不同意我们需要更多信息才能说实话。这样的事情应该有效。另一种方法是使用MINUS关键字来获取第一个查询（x，y和z类中的学生）的所有结果，但第二个查询的结果除外（a，b和c类中的学生））。

选择所有x，y和z类以及a，b和c类的NONE中的所有学生。

SELECT * FROM Students s
WHERE EXISTS (SELECT * FROM Enrollment
    WHERE studentId = s.StudentId AND ClassId = 'x')
AND EXISTS (SELECT * FROM Enrollment
    WHERE studentId = s.StudentId AND ClassId = 'y')
AND EXISTS (SELECT * FROM Enrollment
    WHERE studentId = s.StudentId AND ClassId = 'z')
AND NOT EXISTS (SELECT * FROM Enrollment
    WHERE studentId = s.StudentId AND ClassId IN ('a', 'b', 'c')

Answer 2

假设DDL（仅用于引入名称）：

create table student ( s_id numeric, name varchar );
create table class ( c_id numeric, name varchar );
create table enrollment ( s_id numeric, c_id numeric );

选择不在a，b或c

中的学生的ID

select s_id from enrollments where c_id not in
    (select c_id from class where name in ('a', 'b', 'c');

在x，y或z中选择学生ID

select s_id from enrollments where c_id in
    (select c_id from class where name in ('x', 'y', 'z');

合并

select * from students where
    s_id in (select s_id from enrollments where c_id in
        (select c_id from class where name in ('x', 'y', 'z') and
    s_id not in (select s_id from enrollments where c_id in
        (select c_id from class where name in ('a', 'b', 'c');

SQL Select：同一个表的子集

2 个答案: