SQLite的查询结合使用两个结果集，

Question

这里是我的简单查询：

Select user_id, body 
from Messages where (Messages.author = 'john' and Messages.receipt = 'erick' 
UNION ALL (Messages.author = 'erick' and Messages.receipt = 'john'); 

---

我只是想给这两个简单的SELECT查询相结合，得到的东西，如：

user_id body 
------------------ 
1  hello 
2  hi john 
2  hello this is erick 
1  this is john! 

我在做什么错？

Answer 1

你可以试试：

Select user_id, body 
from Messages where (author = 'john' and receipt = 'erick') or 
(author = 'erick' and receipt = 'john') 

顺便说一句，因为你只从一个表中选择数据，你不必每次你指的是现场时注明其名称。

Answer 2

您可以通过更改WHERE条件做到这一点：

Select body 
from Messages 
    where 
(messages.author = 'john' and messages.receipt = 'erick') 
or 
(messages.author = 'erick' and messages.receipt = 'john') 

Answer 3

您应该使用以前的答案。我认为他们表现更好。

我的回答是我只想澄清UNION关键字：UNION连接两个行集，应使用如下：

-- One rowset: 
SELECT user_id, body FROM Messages WHERE author = 'john' AND receipt = 'erick' 
UNION ALL 
- Another rowset: 
SELECT user_id, body FROM Messages WHERE author = 'erick' AND receipt = 'john' 
; -- Union of both is returned! 

不过，如果你正在寻找一个表中的行避免UNION：刚刚写了一个根据WHERE的条件。