sql - SQL 将表与自身进行比较

Question

Movie(title, year, director, budget, earnings)
Actor(stagename, realname, birthyear)
ActedIn(stagename, title, year, pay)
CanWorkWith(stagename, director)

找到所有成对的艺名（重命名为 stagename1 和 stagename2），使得 stagename1 的演员与 stagename2 的演员在同一部电影中表演，但在该电影中表演的收入较少。

SELECT A.stagename, B.stagename
FROM ActedIn A
LEFT JOIN ActedIn B ON A.title = B.title AND A.year = B.year
          AND B.pay > A.pay
WHERE B.name IS NOT NULL

不确定 WHERE B.name IS NOT NULL 是否有效

Answer 1

http://sqlfiddle.com/#!2/67568/1

select a1.stagename as stagename1, a2.stagename as stagename2, a1.title
from ActedIn as a1
 inner join ActedIn as a2 on (a1.title = a2.title and a1.year = a2.year)
where a1.pay < a2.pay

Answer 2

SELECT A.stagename, B.stagename
FROM ActedIn A
INNER JOIN ActedIn B ON A.title = B.title 
AND A.year = B.year AND B.pay > A.pay
WHERE B.name IS NOT NULL

如果您不想要 B 的 NULL 值，则 LEFT JOIN 将毫无用处。