如何将事物组合映射到关系数据库? [英] How to map combinations of things to a relational database?

问题描述

我有一个表，其记录表示某些对象.为了简单起见，我将假定该表只有一列，这是唯一的ObjectId.现在，我需要一种方法来存储该表中对象的组合.组合必须是唯一的，但可以是任意长度.例如，如果我有ObjectId s

I have a table whose records represent certain objects. For the sake of simplicity I am going to assume that the table only has one column, and that is the unique ObjectId. Now I need a way to store combinations of objects from that table. The combinations have to be unique, but can be of arbitrary length. For example, if I have the ObjectIds

1,2,3,4

我要存储以下组合:

{1,2}, {1,3,4}, {2,4}, {1,2,3,4}

不需要订购.我当前的实现是要有一个将ObjectId s映射到CombinationId s的表Combinations.因此，每个组合都会收到一个唯一的ID:

The ordering is not necessary. My current implementation is to have a table Combinations that maps ObjectIds to CombinationIds. So every combination receives a unique Id:

ObjectId | CombinationId
------------------------
1        | 1
2        | 1
1        | 2
3        | 2
4        | 2

这是上面示例的前两个组合的映射.问题在于，用于查找特定组合的CombinationId的查询似乎非常复杂.该表的两个主要使用方案是遍历所有组合，并检索特定的组合.该表将创建一次，并且永远不会更新.我正在通过JDBC使用 SQLite .有没有更简单的方法或最佳实践来实现这种映射?

This is the mapping for the first two combinations of the example above. The problem is, that the query for finding the CombinationId of a specific Combination seems to be very complex. The two main usage scenarios for this table will be to iterate over all combinations, and the retrieve a specific combination. The table will be created once and never be updated. I am using SQLite through JDBC. Is there any simpler way or a best practice to implement such a mapping?

推荐答案

问题在于，用于查找特定组合的CombinationId的查询似乎非常复杂.

The problem is, that the query for finding the CombinationId of a specific Combination seems to be very complex.

应该不会太糟.如果您希望所有包含所选项目的组合(允许添加其他项目)，则类似于:

Shouldn't be too bad. If you want all combinations containing the selected items (with additional items allowed), it's just something like:

SELECT combinationID
FROM Combination
WHERE objectId IN (1, 3, 4)
GROUP BY combinationID
HAVING COUNT(*) = 3 -- The number of items in the combination

如果您只需要特定的组合(不允许有多余的物品)，它可能会更像:

If you need only the specific combination (no extra items allowed), it can be more like:

SELECT combinationID FROM (
   -- ... query from above goes here, this gives us all with those 3
) AS candidates

-- This bit gives us a row for each item in the candidates, including 
-- the items we know about but also any 'extras'
INNER JOIN combination ON (candidates.combinationID = combination.combinationID)

GROUP BY candidates.combinationID
HAVING COUNT(*) = 3 -- Because we joined back on ALL, ones with extras will have > 3

您还可以在此处(或在原始查询中)使用NOT EXISTS，这似乎更容易解释.

You can also use a NOT EXISTS here (or in the original query), this seemed easier to explain.

最后，您也可以选择一个简单的查询

Finally you could also be fancy and have a single, simple query

SELECT combinationID
FROM Combination AS candidates
INNER JOIN Combination AS allItems ON 
  (candidates.combinationID = allItems.combinationID)
WHERE candidates.objectId IN (1, 3, 4)
GROUP BY combinationID
HAVING COUNT(*) = 9 -- The number of items in the combination, squared

换句话说，如果我们正在寻找{1，2}，并且与{1，2，3}结合使用，我们将得到{candidates，allItems} JOIN结果为:

So in other words, if we're looking for {1, 2}, and there's a combination with {1, 2, 3}, we'll have a {candidates, allItems} JOIN result of:

{1, 1}, {1, 2}, {1, 3}, {2, 1}, {2, 2}, {2, 3}

额外的3个结果导致COUNT(*)在GROUP之后是6行，而不是4行，因此我们知道这不是我们要遵循的组合.

The extra 3 results in COUNT(*) being 6 rows after GROUPing, not 4, so we know that's not the combination we're after.

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