Database 不使用聚合函数的关系代数查询

科目数据库系统的考试任务：

我有以下模式：

Excavator(EID, Type) - EID is a key
Company(Name, HQLocation) - Name is a key
Work(Name, EID, Site, Date) - All collumns together form a key

我必须用关系代数编写这个查询：

"Which company was digging on exactly one site on 1st of May?"

如果没有聚合函数计数，我不知道如何表达它。我知道人们在关系代数中添加了这些函数，但在这次考试中我们被禁止这样做

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您可以使用标准集运算、除法、投影、选择、联接、笛卡尔积。

我现在忘记了正确的关系代数语法，但您可以这样做

       (Worked on >= 1 site on 1st May) 
minus  (Worked on > 1 site on 1st May)
--------------------------------------
equals (Worked on 1 site on 1st May)

下面是一个仅使用注释中提到的运算符并假设重命名的SQL解决方案

SELECT Name
FROM   Work
WHERE  Date = '1st May' /*Worked on at least one site on 1st May */

EXCEPT

SELECT W1.Name /*Worked more than one site on 1st May */
FROM   Work W1
       CROSS JOIN Work W2
WHERE  W1.Name = W2.Name
       AND W1.Date = '1st May'
       AND W2.Date = '1st May'
       AND W2.Site <> W2.Site 

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我假设这将是相对直接的翻译

我现在忘记了正确的关系代数语法，但您可以这样做

       (Worked on >= 1 site on 1st May) 
minus  (Worked on > 1 site on 1st May)
--------------------------------------
equals (Worked on 1 site on 1st May)

下面是一个仅使用注释中提到的运算符并假设重命名的SQL解决方案

SELECT Name
FROM   Work
WHERE  Date = '1st May' /*Worked on at least one site on 1st May */

EXCEPT

SELECT W1.Name /*Worked more than one site on 1st May */
FROM   Work W1
       CROSS JOIN Work W2
WHERE  W1.Name = W2.Name
       AND W1.Date = '1st May'
       AND W2.Date = '1st May'
       AND W2.Site <> W2.Site 

---

我认为这将是相对直接的翻译

负号当然也可以使用。加上并集和交集。对不起，负号当然也可以用。加上并集和交集。很抱歉