分组后成员作为集合两两运算【问题】【回答】

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我是靠谱客的博主漂亮汽车，最近开发中收集的这篇文章主要介绍分组后成员作为集合两两运算【问题】【回答】，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

【问题】

Suppose I have a MySQL database in 1NF with the following tables:

sandwich_id | sandwich_name | sandwich_price
_______________________________________________
0| BLT | 5.5
1| Reuben | 7.0
3| Grilled Cheese | 3.75
...

and a separate table that stores all the ingredient values:

sandwich_id | ingredient
__________________________
0| bacon
0| lettuce
0| tomato
1| corned beef
1| swiss cheese
...

How can I compare all the sandwiches by their ingredients to determine which are the most similar?

(Also, is there a technical term for that second table that I’m missing? I want to call it a map table, but I know that’s not quite right, since a map table stores foreign keys for two tables and this one’s more of an offshoot of the first…)

【回答】

把不同 sandwich 的 ingredient 集合两两做交集，计算交集成员数量再排序即可。但 SQL 没有显式集合，代码很难懂。这种数据量不大时用 SPL 更方便：

	A
1	$select i.sandwich_id sandwich_id, i.ingredient ingredient, s.sandwich_name sandwich_namefrom ingredientTable i,sandwichTable s where i.sandwich_id=s.sandwich_id
2	=A1.group(sandwich_id;sandwich_name,~.(ingredient):collection)
3	=xjoin(A2;A2).select(_1.sandwich_id<_2.sandwich_id)
4	=A3.new((_1.collection ^ _2.collection).len():sameCount,_1.sandwich_name,_2.sandwich_name)
5	=A4.sort(-sameCount)

A2: 将 A1 按 sandwich_id 分组。collection 是每种 sandiwich 的 ingredient 集合。

A3: 将 sandwich 两两分为一组。比如 0,1,3 可分为三组：[0,1],[0,3],[1,3]

A4: 求各组交集的成员数量。“^”表示求交集。

A5: 将 A4 的结果集降序排序

关于显式集合请参考【集算器的集合思维】。更多例子可参考【集算器的集合运算举例】