Question

我的问题如下：

SELECT title, 'dossier' as Source FROM dossier  
UNION ALL 
SELECT title, 'contract' as Source FROM contract ORDER BY `time` LIMIT 5

两个表中都存在

time列，但MySQL会引发以下错误：

未知专栏＆＃39; time＆＃39;在＆＃39;订单条款＆＃39;

当我删除, 'dossier' as Source和, 'contract' as Source时，查询可以正常运行。

Answer 1

order by子句适用于此处的union all整体选择，其中没有time列（仅title和Source）。你可以做的是使用临时表：

select `title`, `source` from (
  select `title`, 'dossier' as `Source`, `time` from dossier  
  union all 
  select `title`, 'contract', `time` from contract
) tbl 
order by `time` 
limit 5

Answer 2

@Potashin有一种解决方法。

您应该了解order by不是select子句的一部分。它只知道正在选择的列。

另一种解决方案很简单。。。这只是在结果集中包含time。如果使用括号，可能会更清楚：

(SELECT title, 'dossier', time as Source
 FROM dossier  
)
UNION ALL 
(SELECT title, 'contract', time as Source
 FROM contract
)
ORDER BY `time`
LIMIT 5;

我应该注意，如果表格很大并且time上有索引，那么以下内容可能更有效：

(SELECT title, 'dossier', time as Source
 FROM dossier  
 ORDER BY `time`
 LIMIT 5
)
UNION ALL 
(SELECT title, 'contract', time as Source
 FROM contract
 ORDER BY `time`
 LIMIT 5
)
ORDER BY `time`
LIMIT 5;

ORDER BY子句不适用于UNION ALL

2 个答案: