Mysql 分组依据、分组条件、条件以及如何选择

我有三张桌子

objects     id, field1, field2, ...
groups      id, title, color
group_ref   oid, gid

因此，每个对象都可以在n个组中

我想显示一个对象列表，该列表的一个字段是对象所在的组。下面是我的想法：

SELECT o.id,
CONCAT_WS("||", GROUP_CONCAT(gx.gid separator "|"), GROUP_CONCAT(gr.title, "#", gr.color separator "|")) AS groups 
FROM objects AS o 
LEFT JOIN group_ref AS gx ON o.id = gx.oid 
LEFT JOIN groups AS gr ON gx.gid = gr.id 
WHERE 1 
GROUP BY o.id

这实际上是可行的，我可以从（连接的）组字段中创建所需的接口

问题是：如何选择仅在一个特定组中的对象（例如gid=4）

这只能得到部分结果：

WHERE gr.id = 4

或

非常感谢您的帮助！（也许还有一种更为优雅的方式来制作concat thingy）

这行吗

WHERE gr.id = 4
GROUP BY o.id
HAVING COUNT(*) = 1

---

这应仅显示一个组中的对象：

SELECT o.id,
CONCAT_WS("||", GROUP_CONCAT(gx.gid separator "|"),
    GROUP_CONCAT(gr.title, "#", gr.color separator "|")) AS groups 
FROM objects AS o 
LEFT JOIN group_ref AS gx ON o.id = gx.oid 
LEFT JOIN groups AS gr ON gx.gid = gr.id
GROUP BY o.id
HAVING COUNT(DISTINCT gx.gid) = 1

---

如果我理解正确，您希望获得组4中的所有对象以及这些对象所在的所有组。您希望在group_ref表上加入两次，一次用于筛选（常规加入），一次用于获取相关组（左加入）

---

“不起作用”不是一个看起来很有趣的解释，但它让我了解了所有只在一（1）组中的对象。我需要获取特定组中的所有对象（例如gid 4）。

SELECT o.id,
CONCAT_WS("||", GROUP_CONCAT(gx.gid separator "|"),
    GROUP_CONCAT(gr.title, "#", gr.color separator "|")) AS groups 
FROM objects AS o 
LEFT JOIN group_ref AS gx ON o.id = gx.oid 
LEFT JOIN groups AS gr ON gx.gid = gr.id
GROUP BY o.id
HAVING COUNT(DISTINCT gx.gid) = 1

SELECT o.id,
CONCAT_WS("||", GROUP_CONCAT(gx.gid separator "|"),
GROUP_CONCAT(gr.title, "#", gr.color separator "|")) AS groups 
FROM objects AS o 
JOIN group_ref AS gfilter ON o.id = gfilter.oid AND gfilter.gid=4
LEFT JOIN group_ref AS gx ON o.id = gx.oid 
LEFT JOIN groups AS gr ON gx.gid = gr.id 
WHERE 1 
GROUP BY o.id