＃1222-使用的SELECT语句的列数不同

Question

为什么我得到＃1222-使用的SELECT语句具有不同数量的列？我正在尝试从此用户朋友和他自己处加载墙贴。

SELECT u.id AS pid, b2.id AS id, b2.message AS message, b2.date AS date FROM 
(
    (
        SELECT b.id AS id, b.pid AS pid, b.message AS message, b.date AS date FROM 
        wall_posts AS b 
        JOIN Friends AS f ON f.id = b.pid 
        WHERE f.buddy_id = '1' AND f.status = 'b'
        ORDER BY date DESC
        LIMIT 0, 10
    )
    UNION
    (
        SELECT * FROM
        wall_posts
        WHERE pid = '1'
        ORDER BY date DESC
        LIMIT 0, 10
    )
    ORDER BY date DESC
    LIMIT 0, 10
) AS b2 
JOIN Users AS u
ON b2.pid = u.id
WHERE u.banned='0' AND u.email_activated='1'
ORDER BY date DESC
LIMIT 0, 10

wall_posts表结构如下所示 id date privacy pid uid message

Friends表结构如下所示 Fid id buddy_id invite_up_date status

pid代表配置文件ID。我不太确定发生了什么。

Answer 1

UNION中的第一条语句返回四列：

SELECT b.id AS id, 
       b.pid AS pid, 
       b.message AS message, 
       b.date AS date 
  FROM wall_posts AS b

第二个返回 六 ，因为*扩展为包括以下所有列WALL_POSTS：

SELECT b.id, 
       b.date, 
       b.privacy,
       b.pid. 
       b.uid message
  FROM wall_posts AS b

在UNION与UNION ALL运营商要求：

1. 组成UNION查询的所有语句中存在相同数量的列
2. 数据类型必须在每个位置/列都匹配

使用：

FROM ((SELECT b.id AS id, 
             b.pid AS pid, 
             b.message AS message, 
             b.date AS date 
        FROM wall_posts AS b 
        JOIN Friends AS f ON f.id = b.pid 
       WHERE f.buddy_id = '1' AND f.status = 'b'
    ORDER BY date DESC
       LIMIT 0, 10)
      UNION
      (SELECT id,
              pid,
              message,
              date
         FROM wall_posts
        WHERE pid = '1'
     ORDER BY date DESC
        LIMIT 0, 10))