MySQL查询从多个表返回包含OR的关系

Question

我有两个实体，“用户”和“产品”，以及两个表：一个用于存储哪些用户已查看哪些产品，另一个用于存储哪些用户喜欢哪些产品。我想查询一次并返回结果如下图所示：

表“视图”：

表“喜欢”：

查询结果：

理想情况下，结果中不应重复user_id，product_id。请注意，如果用户既不查看也不喜欢某个产品，则不应该返回它（即，不应该返回FALSE，FALSE行）。

由于MySQL不支持FULL JOIN，我怀疑应该有一个使用UNION ALL的解决方法。我很欣赏任何想法。

Answer 1

你可以尝试一下：

SELECT user_id, product_id, max(`view`) `view`, max(`like`) `like` FROM (
  SELECT user_id, product_id, TRUE `view`, FALSE `like` FROM views
  UNION ALL
  SELECT user_id, product_id, FALSE, TRUE FROM likes
) s
GROUP BY user_id, product_id

顺便说一下，你应该尽量避免使用view和like作为名称，因为它们是MySQL中的保留字。

请记住，在MySQL中true和1以及false和0都是同义词。

小提琴here。

Answer 2

SELECT 
    v.User_ID, v.Product_ID, true AS `View`, IF (l.User_ID, true, false) AS `Like` 
FROM views v
LEFT JOIN likes l ON (l.User_ID = v.User_ID AND l.Product_ID = v.Product_ID)
UNION
SELECT 
    l.User_ID, l.Product_ID, IF (v.User_ID, true, false) AS `View`, true AS `Like` 
FROM likes l
LEFT JOIN views v ON (l.User_ID = v.User_ID AND l.Product_ID = v.Product_ID)
ORDER BY User_ID, Product_ID

Answer 3

还有一种方法：

SELECT user_id, product_id, 
       'TRUE' AS view_result, 
       CASE WHEN l.user_id IS NULL THEN 'FALSE' ELSE 'TRUE' END AS like_result 
FROM views AS v
  NATURAL LEFT JOIN likes AS l
UNION ALL
SELECT user_id, product_id, 
       'FALSE', 'TRUE'
FROM views AS v
  NATURAL RIGHT JOIN likes AS l
WHERE v.user_id IS NULL ; 

经过测试：SQL-Fiddle

Answer 4

SELECT
    User_ID,
    Product_ID,
    Views.User_ID IS NOT NULL AS View,
    Likes.User_ID IS NOT NULL AS Like
FROM
    Users
    LEFT JOIN Views USING (User_ID)
    LEFT JOIN Likes USING (User_ID)
WHERE
    Views.User_ID IS NOT NULL
    OR Likes.User_ID IS NOT NULL