从关系型代数的三个关系中选择 [英] Selecting from three Relation with Relational Algebra

问题描述

我有此表

Customer{cid, name, phone, address}
Employee{eid, name, positon, salary}
Resp_for{cid,eid}

现在我想在关系代数中询问是否应打印出所有可为客户使用的员工：'Obama'

Now I want to ask in relational Algebra that it should print out all employees that are resposible for the Customer: 'Obama'

我正在尝试，但是不确定我是否清楚地理解关系代数，所以我创建了这个文件：

I am trying it but not sure that I understand the relational algebra clearly, so I create this:

PROJECT   (SELECT                 (Customer x Employeee x Resp_for))
       E.eid       C.name = 'Obama'
                 AND R.cid = C.cid
                 And R.eid = E.eid

那么它如何像sql查询一样？

so how it looks as sql query?

SELECT E.eid
FROM Customer JOIN Employee JOIN Resp_for
WHERE C.name = 'Obama'
      AND R.cid = C.cid
      And R.eid = E.eid

是正确的吗？

推荐答案

正确的ugh如果不将表别名（R，C和E）放在各自的表上，就不太可能运行。

That is correct, though without putting the table aliases (R, C and E) against their respective tables it is unlikely to run.

有一种新的表示联接的形式，这种形式更常见这些天：

There is a new form for expressing joins which is more common these days:

SELECT E.eid
FROM Customer C
INNER JOIN Resp_for R
    ON R.cid = C.cid
INNER JOIN Employee E
    ON E.eid = R.eid
WHERE C.name = 'Obama'

相同的含义，不同的格式和恕我直言易于吸收。

Same meaning, different format and IMHO easier to assimilate.

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