使用的SELECT語句具有不同數量的列（REDUX !!）

Question

那里還有另一個與此類似的問題，但似乎沒有回答我的問題。

我的問題是：為什么我會找回此錯誤ERROR 1222 (21000): The used SELECT statements have a different number of columns與以下SQL ERROR 1222 (21000): The used SELECT statements have a different number of columns

SELECT * FROM friends
LEFT JOIN users AS u1 ON users.uid = friends.fid1
LEFT JOIN users AS u2 ON users.uid = friends.fid2
WHERE (friends.fid1 = 1) AND (friends.fid2 > 1)
UNION SELECT fid2 FROM friends
WHERE (friends.fid2  = 1) AND (friends.fid1 < 1)
ORDER BY RAND()
LIMIT 6;

這里的users ：

+------------+---------------+------+-----+---------+----------------+
| Field      | Type          | Null | Key | Default | Extra          |
+------------+---------------+------+-----+---------+----------------+
| uid        | int(11)       | NO   | PRI | NULL    | auto_increment |
| first_name | varchar(50)   | NO   |     | NULL    |                |
| last_name  | varchar(50)   | NO   |     | NULL    |                |
| email      | varchar(128)  | NO   | UNI | NULL    |                |
| mid        | varchar(40)   | NO   |     | NULL    |                |
| active     | enum('N','Y') | NO   |     | NULL    |                |
| password   | varchar(64)   | NO   |     | NULL    |                |
| sex        | enum('M','F') | YES  |     | NULL    |                |
| created    | datetime      | YES  |     | NULL    |                |
| last_login | datetime      | YES  |     | NULL    |                |
| pro        | enum('N','Y') | NO   |     | NULL    |                |
+------------+---------------+------+-----+---------+----------------+

這是friends ：

+---------------+--------------------------------------+------+-----+---------+----------------+
| Field         | Type                                 | Null | Key | Default | Extra          |
+---------------+--------------------------------------+------+-----+---------+----------------+
| friendship_id | int(11)                              | NO   | MUL | NULL    | auto_increment |
| fid1          | int(11)                              | NO   | PRI | NULL    |                |
| fid2          | int(11)                              | NO   | PRI | NULL    |                |
| status        | enum('pending','accepted','ignored') | NO   |     | NULL    |                |
+---------------+--------------------------------------+------+-----+---------+----------------+

如果您也想對這里發生的任何瘋狂事情提供任何反饋，請隨時這樣做。 我會結成小塊。

Answer 1

UNION（ UNION和UNION ALL ）要求所有被UNION ALL的查詢都具有：

1. SELECT子句中的列數相同
2. 列數據類型必須在每個位置都匹配

您的查詢有：

SELECT f.*, u1.*, u2.* ...
UNION 
SELECT fid2 FROM friends

我最簡單的重寫是：

   SELECT f.*, u.*
     FROM FRIENDS AS f
     JOIN USERS AS u ON u.uid = f.fid2
    WHERE f.fid1 = 1 
      AND f.fid2 > 1
UNION 
   SELECT f.*, u.*
     FROM FRIENDS AS f
     JOIN USERS AS u ON u.uid = f.fid1
    WHERE f.fid2  = 1 
      AND f.fid1 < 1
ORDER BY RAND()
LIMIT 6;

您已經兩次兩次加入USERS表，但似乎沒有使用該信息。