Question

$queryActivities = mysql_query("
SELECT ua.status, ua.date, 'status' AS is_table FROM users_statuslog ua
   WHERE ua.uid = '{$showU[id]}'
UNION ALL 
SELECT us.message, us.date 'wall' FROM users_wall us 
   WHERE us.uid = '{$showU[id]}'
ORDER BY `date` DESC");

这就是我现在所拥有的。而且我需要来自users_wall的更多列，比在users_statuslog中需要的更多。我该如何选择它们？因为我不能这样做：

（假设我也想要isReplyFrom并从users_wall查看）

$queryActivities = mysql_query("
SELECT ua.status, ua.date, 'status' AS is_table FROM users_statuslog ua
   WHERE ua.uid = '{$showU[id]}'
UNION ALL 
SELECT us.message, us.date, us.isReplyFrom, us.viewed 'wall' FROM users_wall us 
   WHERE us.uid = '{$showU[id]}'
ORDER BY `date` DESC");

我得到了：

使用的SELECT语句具有不同的列数。

Answer 1

如消息所示，您有两个选择，列数不同。
联合会将第一个选择的结果“连接”到第二个选择，但如果列数不相同则无法工作。

要么找到一种方法来拥有相同数量的列，要么在您的请求中添加虚拟列：

SELECT ua.status, ua.date, 'dummy' AS replyFrom, 'dummy' AS viewed, 'status' AS is_table FROM users_statuslog ua
    WHERE ua.uid = '{$showU[id]}'
UNION ALL
SELECT us.message, us.date, us.isReplyFrom, us.viewed, 'wall' FROM users_wall us 
    WHERE us.uid = '{$showU[id]}'
ORDER BY `date` DESC

资源：

w3school.com - SQL UNION operator

Answer 2

您需要在首次查询中添加一些假列

$queryActivities = mysql_query("
SELECT ua.status, ua.date, null AS isReplyFrom, null AS viewed, 'status' AS is_table FROM users_statuslog ua
   WHERE ua.uid = '{$showU[id]}'
UNION ALL 
SELECT us.message, us.date, us.isReplyFrom, us.viewed, 'wall' FROM users_wall us 
   WHERE us.uid = '{$showU[id]}'
ORDER BY `date` DESC");

Answer 3

使用SELECT时，UNION语句的列数不能相同。您可能希望添加常量字段来代替额外列。也许是这样的：

SELECT ua.status, ua.date, 0 AS isReplyFrom, 'status' AS is_table 
FROM   users_statuslog ua
WHERE ua.uid = '{$showU[id]}'
UNION ALL 
SELECT us.message, us.date, us.isReplyFrom, us.viewed 'wall' 
FROM users_wall us 
WHERE us.uid = '{$showU[id]}'
ORDER BY `date` DESC

在UNION中选择列

3 个答案: