Question

这是查询：

SELECT a.id, a.userName,if(o.userId=1,'C',if(i.userId=1,'I','N')) AS relation 
       FROM tbl_users AS a 
       LEFT JOIN tbl_contacts AS o ON a.id = o.contactId 
       LEFT JOIN tbl_invites AS i ON a.id = i.invitedId 
  ORDER BY relation

返回输出如下：

+----+--------------+-------------+
| ID | USERNAME     | RELATION    | 
+----+--------------+-------------+
|  1 |          ray |           C | 
+----+--------------+-------------+
|  2 |         john |           I | 
+----+--------------+-------------+
|  1 |         ray  |           N | 
+----+--------------+-------------+

我需要通过检查id是否可能重复来从select查询中删除第三行。优先顺序如下：

C - ＆gt;我 - ＆gt; N.所以既然已经有了一张带有C的“光线”，我再也不想用I或N了。

我尝试添加distinct（a.id）但它不起作用。我该怎么做呢？

为什么DISTINCT不适用于此？

Answer 1

根据您提供的规格，您所要做的就是按ID和用户名分组，然后选择您可以找到的最低关系值（自C < I < N起）

SELECT a.id, a.userName, MIN(if(o.userId=1,'C',if(i.userId=1,'I','N'))) AS relation 
    FROM tbl_users AS a 
    LEFT JOIN tbl_contacts AS o ON a.id = o.contactId 
    LEFT JOIN tbl_invites AS i ON a.id = i.invitedId 
GROUP BY a.id, a.username

Answer 2

如您在此manual page中所见，有多种方法可以获得分组最大/最小值。

最适合你的是第一个，如果行的顺序不能按字母顺序定义。

在这种情况下，如果所需的顺序是z-a-m（参见Rams＆＃39;评论），则需要FIELD()函数。

所以你的答案是

SELECT 
a.id, 
a.userName,
if(o.userId=1,'C',if(i.userId=1,'I','N')) AS relation 
FROM tbl_users a
LEFT JOIN tbl_contacts AS o ON a.id = o.contactId 
LEFT JOIN tbl_invites AS i ON a.id = i.invitedId 
WHERE 
if(o.userId=1,'C',if(i.userId=1,'I','N')) = (
    SELECT 
    if(o.userId=1,'C',if(i.userId=1,'I','N')) AS relation 
    FROM tbl_users aa
    LEFT JOIN tbl_contacts AS o ON aa.id = o.contactId 
    LEFT JOIN tbl_invites AS i ON aa.id = i.invitedId 
    WHERE aa.id = a.id AND aa.userName = a.userName
    ORDER BY FIELD(relation, 'N', 'I', 'C') DESC 
    LIMIT 1
)

注意，您也可以像ORDER BY FIELD(relation, 'C', 'I', 'N')那样使其更具可读性/直观性。我反过来说，因为如果你有可能有一个＆＃39; X＆＃39;在关系中，FIELD（）函数将返回0，因为X未指定为参数。因此，它将在C＆＃39;之前进行排序。通过排序降序和转动参数的顺序，这不可能发生。

当与另一个表

2 个答案: