SQL case语句-根据其他2个表从第三个值中选择值

Question

我有2个数据库db1和db2-

db1-movies和tickets中有2个表。 db2-boxoffice中有1个表。

我想在获取结果时应用以下逻辑-

if boxoffice.movie_id == movie.id
  tickets.movie_id = boxoffice.movie_id
else
  tickets.movie_id = movie.id
end

认为票证表与此类似

我写了这样的东西，但是不完整-

select mvoie_id from tickets-- query that i was using...some syntax might be wrong but this is to give you the idea of what i was trying to do (select case when db2.boxoffice.movie_id = db1.movie.id then db1.tickets.movie_id= db2.boxoffice.movie_id else db1.tickets.movie_id = db1.movie.id end)

预期结果-

Tickets table

id movie_id

由于我无法构建查询，有人可以帮助查询吗？

Answer 1

SELECT tickets.* (
  SELECT (
    CASE 
      WHEN movies.id IS NOT NULL THEN boxoffice.movie_id
      ELSE movies.id
    END
  ) AS movie_id
  FROM movies
  LEFT JOIN boxoffice ON boxoffice.movie_id = movies.id
) AS data_table
JOIN tickets ON tickets.movie_id = data_table.movie_id

说明：

WHEN movies.id IS NOT NULL THEN boxoffice.movie_id，检查是否存在与条件if if boxoffice.movie_id == movie.id类似的匹配，否则，忽略这些行并仅考虑movie.id

Answer 2

如果我理解正确，可以尝试一下。

SELECT t2.*
FROM (
    SELECT b.movie_id
    FROM boxoffice b JOIN movie m
    ON b.movie_id = m.id
    UNION ALL
    SELECT m.id
    FROM boxoffice b JOIN movie m
    ON b.movie_id <> m.id
) t1 JOIN tickets t2 ON t1.movie_id = t2.movie_id

或者您可以尝试在郊区使用LEFT JOIN

SELECT t2.*
FROM (
    SELECT (CASE WHEN m.id IS NOT NULL THEN b.movie_id ELSE m.id END) movie_id
    FROM boxoffice b LEFT JOIN movie m
    ON b.movie_id = m.id
) t1 JOIN tickets t2 ON t1.movie_id = t2.movie_id