SQL查询仅适用于不同组的所有元素对

Question

我有一个名为Survey的表,其中包含一个Group Column和一个Subject Column

CREATE TABLE survey (
  `group` INT NOT NULL,
  `subject` VARCHAR(16) NOT NULL,
  UNIQUE INDEX (`group`, `subject`)
);

INSERT INTO survey 
  VALUES
  (1, 'sports'),
  (1, 'history'),
  (2, 'art'),
  (2, 'music'),
  (3, 'math'),
  (3, 'sports'),
  (3, 'science')
;

我试图找出一个查询,它将返回不属于同一组的所有主题对.所以从上面的例子中,我希望看到这些对在表中返回:

science - history  
science - art  
science - music  
history - math  
sports  - art  
sports  - music  
history - art  
history - music

因此,查询不应返回:

sports - history  

作为一个例子,因为他们都在第1组.

非常感谢.

Answer 1

SELECT s1.subject,
       s2.subject
FROM   survey s1
       JOIN survey s2
         ON s1.subject < s2.subject
GROUP  BY s1.subject,
          s2.subject
HAVING COUNT(CASE
               WHEN s1.groupid = s2.groupid THEN 1
             END) = 0