在不使用关系代数中的max函数的情况下寻找两个表的最大值

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问题描述

假设我有两个表A{int m}和B{int m}，我必须在使用关系代数的两个表中找到最大的 m 使用加入我们可以做到，但我不确定我的猜测是否正确.

注意:这是一个面试问题.

推荐答案

嗯，我很困惑这个问题为什么涉及两张表.对于问的问题，我只会将两者结合(如Stilescrisis所做的那样)，然后解决一个桌子.

so:如何仅使用natjoin在表中找到最大m?这是在表上找到层次结构的表上的顶级节点的简化版本(想想汇编/组件爆炸或组织图).

关键想法是，我们需要将表"复制"到具有不同属性名称的东西中，以便我们可以对元组进行比较. (因此，这将使用natjoin aka交叉产品的退化形式).请参阅此处的示例如何找到具有关系代数的最大值?

A NOT MATCHING
((A x (A RENAME m AS mm)) WHERE m < mm)

亚trahend的所有元素都比其他一些元组少.反加入除外的所有元组 - 即最大. (我认为使用不匹配比负相比更易于理解，并且不需要关系与工会兼容.它大致相当于不存在SQL)

[我已经使用了教程D语法，以避免用希腊字母弄乱.]

其他推荐答案

SELECT M FROM (SELECT M FROM A UNION SELECT M FROM B) ORDER BY M DESC LIMIT 1

这不使用MAX，只是普通的香草SQL.

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问题描述

Suppose I have two tables A{int m} and B{int m} and I have to find maximum m among two tables using relational algebra but I cannot use max function.How can I do it?I think using join we can do it but i am not sure if my guess is correct or not.

Note: this is an interview question.

推荐答案

Hmm, I'm puzzled why the question involves two tables. For the question as asked, I would just UNION the two (as StilesCrisis has done), then solve for a single table.

So: how to find the maximum m in a table using only NatJOIN? This is a simplified version of finding the top node on a table that holds a hierarchy (think assembly/component explosions or org charts).

The key idea is that we need to 'copy' the table into something with a different attribute name so that we can compare the tuples pair-wise. (And this will therefore use the degenerate form of NatJOIN aka cross-product). See example here How can I find MAX with relational algebra?

A NOT MATCHING
((A x (A RENAME m AS mm)) WHERE m < mm)

The subtrahend is all tuples with m less than some other tuples. The anti-join is all the tuples except those -- ie the MAX. (Using NOT MATCHING I think is both more understandable than MINUS, and doesn't need the relations to be UNION-compatible. It's roughly equivalent to SQL NOT EXISTS).)

[I've used Tutorial D syntax, to avoid mucking about with greek letters.]

其他推荐答案

SELECT M FROM (SELECT M FROM A UNION SELECT M FROM B) ORDER BY M DESC LIMIT 1

This doesn't use MAX, just plain vanilla SQL.

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