mysql - MySQL查询从多个表返回OR-inclusive关系

Question

我有两个实体，“用户”和“产品”，以及两个表：一个存储哪些用户查看了哪些产品，另一个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，结果中不应有重复的 user_id、product_id。请注意，如果用户既没有查看也没有喜欢产品，则不应返回该产品（即，不应有 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

你可以试试这个：

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

顺便说一句，您应该尽量避免使用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，1以及false和0是同义词。

在这里拉小提琴。

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

另一种方法：

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 ; 

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