SQL 找到所有同源类别

Question

我在类别和项目之间有一个多对多关系，它是用 "category_item" table 实现的，有 2 列（PK）：category_id 和 item_id.

两个同源类别有相同的项目（顺序无关）：

A
  1
  2

与

同源

B
  1
  2

但不是

C       D       E
  1       1       1
  3               2
                  3

给定类别 ID，如何找到所有同源类别？

假设这个数据在item_category:

我想找到与A同源的所有类别（预期结果只是B）

我目前正在尝试：

select r2.category_id
from category_item r1, category_item r2
where r1.category_id = ?
    and r2.category_id <> r1.category_id
    and r1.item_id = r2.item_id

构建 table:

A   1   B   1
A   1   C   1
A   1   D   1
A   1   E   1
A   2   B   2
A   2   D   2
A   2   E   2

但我不知道如何继续...

我正在使用 MySQL 5.7，但我只想使用通用的 SQL。

请注意，这不是作业（我也不认为任何老师会布置这么复杂的作业），它只是针对现实世界问题的一个额外简化的用例

Answer 1

为什么不使用 Group_Concat() 然后分组？

select ColA, group_concat(ColB order by ColB separator '|') as concat_line
from CategoryItem
group by ColA

如果你把它停在某个地方，你可以将它与它本身进行比较。

Answer 2

您也可以使用子查询和 exists/not exists

Select Distinct category_id 
from category_item ci
Where not exists   -- this allows only cats that do not have all req items
      (select * from category_item
       Where category_id = ci.category_id 
         and item_id Not in 
            (Select item_id from category_item
             Where category_Id = @catId))
  and not exists   -- this filters out cats that have xtra items
      (Select * from category_item
       Where category_Id = @catId
          and item_id Not in
            (Select item_id from category_item
             Where category_Id = ci.category_Id )) 
  and category_Id <> @catId -- <- categoryId of category you are matching
                            -- this line filters out the category you are 
                            -- matching against. Remove it if you want all
                            -- homologous categories

解释：

Select All distinctCategory_Ids from the join table where: First subquery: where there is not another join table row for that same category with anItem_idthat is *not* in the set ofItem_ids for the category you are matching against, and Second subquery: where there is not an item in the category you are matching against that is not also in the category you are selecting (ci table)

SQL 找到所有同源类别

SQL find all homologous categories

mysql

sql

cross-join

给定类别 ID，如何找到所有同源类别？