SQL Select：同一個表的子集

Question

我有三個相關的表（學生，班級，招生）（一個學生可以有多個註冊，一個班級可以有多個註冊），我需要從中選擇所有註冊班級的學生（x，y，z），但不在類（a，b，c）中。我怎樣才能構建我的查詢來實現這一目標？

Answer 1

我不同意我們需要更多的信息才能說實話。像這樣的東西應該工作。另一種方法是使用關鍵字MINUS來獲得第一個查詢（類x，y和z中的學生）的所有結果，但第二個查詢（類a，b和c中的學生）的結果除外。

選擇所有x，y和z類以及類別a，b和c中的所有類的所有學生。

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

應該DDL（只引進名）：

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); 

1. 學生選擇id，而不是在A，B或C

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

2. 選擇學生的IDS x，y或z

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

3. 結合起來

   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');