Question

我有三张桌子

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

所以每个对象都可以在n组中。

我想显示一个对象列表，该列表中的一个字段是一个对象所在的组。所以这就是我想出的：

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

这实际上有效，我可以在（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）？

这只是结果的一部分：

WHERE gr.id = 4

或

HAVING gr.id = 4

任何帮助非常感谢！（也许有一种更优雅的方式可以用于concat thingy）

Answer 1

如果我理解正确，你想获得第4组中的所有对象以及那些对象所在的所有组。你想在group_ref表上加入两次，一次过滤（常规JOIN）和一次得到相关小组（LEFT JOIN）。

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

Answer 2

这有用吗？

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

Answer 3

这应显示仅在一个组中的对象：

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

GROUP BY，GROUP_CONCAT，CONCAT以及如何选择

3 个答案: