Question

我有三张桌子：

用户（所有用户）
朋友（关系表由ID1和ID2列组成，基本上可以告诉谁的朋友）
帖子（所有帖子都包含已写入用户的ID）。

我想选择用户在posts表中写的所有帖子以及朋友表指定的朋友写的帖子，我写了这个查询：

SELECT
posts.ID as ID,
CONCAT(users.FirstName, ' ', users.LastName) as Name,
posts.Date as Date,
posts.Text as Text
FROM users, posts, friends
WHERE users.ID = posts.UserID AND
((posts.UserID = friends.ID1 AND friends.ID2 = '10000007') OR 
(posts.UserID = friends.ID2 AND friends.ID1 = '10000007')
OR (posts.UserID = '10000007'))

这给了我朋友写的帖子，里面有许多额外的行，我不知道它来自哪里。那么有人能告诉我查询中的问题在哪里吗？

Answer 1

就我个人而言，我认为使用这种类型的选择语法（SELECT ... FROM table1，table2，table3）有点令人困惑，如果你看到一些奇怪的行为，我就不会怀疑它。怎么样呢：

SELECT DISTINCT
posts.ID as ID,
CONCAT (users.FirstName, ' ', users.LastName) as Name,
posts.Date as Date,
posts.Text as Text
FROM
users
LEFT JOIN posts ON users.ID = posts.UserID
LEFT JOIN friends ON (
  (posts.UserID = friends.ID1 AND friends.ID2 = '10000007') OR
  (posts.UserID = friends.ID2 AND friends.ID1 = '10000007'))
HAVING
posts.UserID IS NOT NULL OR friends.ID1 IS NOT NULL

Answer 2

我会这样写。

SELECT posts.ID as ID,
       CONCAT(users.FirstName, ' ', users.LastName) as Name,
       posts.Date as Date,
       posts.Text as Text

FROM  users

      LEFT JOIN posts 
      ON users.ID = posts.UserID
      OR posts.UserID IN 
      (SELECT ID2 FROM friends WHERE ID1 = '10000007' 
       UNION 
       SELECT ID1 FROM friends WHERE ID2 = '10000007')

WHERE  users.ID = '10000007'

Answer 3

如果您使用IN，则无需嵌套加入好友表和用户表：

SELECT
    posts.ID as ID,
    users.ID as UserID,
    CONCAT(users.FirstName, ' ', users.LastName) as Name,
    posts.Date as Date,
    posts.Text as Text
FROM users, posts -- Try to change the old style join to 
                  -- FROM users JOIN posts 
                  -- ON users.ID = posts.UserID Where ...
WHERE users.ID = posts.UserID 
AND (posts.UserID = '10000007' OR
    posts.UserID IN 
        (
        SELECT ID 
        FROM friends
        WHERE friends.ID1 = '10000007' 
        UNION
        SELECT ID 
        FROM friends
        WHERE friends.ID2 = '10000007' 
        )
    )

您的原始查询将返回重复的行，因为您根据一对多关系加入了朋友表。你有两个选择来改变它。一种方法是在select语句中添加distinct。另一种方式是使用这样的存在（如果用户有太多恶魔，EXISTS将比IN更有效率：

SELECT
    posts.ID as ID,
    CONCAT(users.FirstName, ' ', users.LastName) as Name,
    posts.Date as Date,
    posts.Text as Text
FROM users
JOIN posts
ON   users.ID = posts.UserID 
WHERE 
    EXISTS
       (SELECT 1 
        FROM friends
           (posts.UserID = friends.ID1 AND friends.ID2 = '10000007') 
           OR 
           (posts.UserID = friends.ID2 AND friends.ID1 = '10000007')
       )
     OR posts.UserID = '10000007'

Answer 4

我能够使用嵌套的SELECT语句解决我的问题：

SELECT
posts.ID as ID,
users.ID as UserID,
CONCAT(users.FirstName, ' ', users.LastName) as Name,
posts.Date as Date,
posts.Text as Text
FROM users, posts
WHERE users.ID = posts.UserID AND
(posts.UserID = '10000007' OR
posts.UserID IN (
SELECT users.ID 
FROM users, friends
WHERE (friends.ID1 = '10000007' AND users.ID = friends.ID2)
OR (friends.ID2 = '10000007' AND users.ID = friends.ID1)
))

来自MySql查询的奇怪结果

4 个答案: