Question

我有一个mysql查询：

Select count(*) as cnt from  page06 where dl = 1 and TYPE = 'mempage' union 
Select count(*) as cnt from  page06 where err = 1 and TYPE = 'mempage' union  
Select count(*) as cnt from  page06 where dl = 1 and err=1 and TYPE = 'mempage'

在大多数情况下，dl和err列为0，因此最后两个选择将返回0。我发现这个查询只返回一个不符合我预期的查询。我找到了这个解决方案：

Select count(*) as cnt,'s' as p from  page06 where dl = 1 and TYPE = 'mempage' union 
Select count(*),'d' as p  from  page06 where err = 1 and TYPE = 'mempage' union  
Select count(*),'f' as p   from  page06 where dl = 1 and err=1 and TYPE = 'mempage'

此查询工作正常但我已编写第一种格式的代码。我想知道为什么会发生这种情况，是否有任何解决方案（除了我所说的）来解决这个问题？
我不想使用我的解决方案的原因是我有很多书面查询，我不想（甚至不能）改变它们！我正在寻求的是设置为连接的选项之王（如set names 'latin1';）

Answer 1

使用UNION ALL代替UNION

查看此帖子，了解有关差异的详细说明： What is the difference between UNION and UNION ALL?

SELECT count(*) AS cnt FROM  page06 WHERE dl = 1 AND TYPE = 'mempage' UNION ALL
SELECT count(*) AS cnt FROM  page06 WHERE err = 1 AND TYPE = 'mempage' UNION ALL
SELECT count(*) AS cnt FROM  page06 WHERE dl = 1 AND err=1 AND TYPE = 'mempage'

mysql union不能按预期工作

1 个答案: