Question

我试图将SQL命令转换为关系代数。最大的问题是，有一个＆＃34; NOT IN＆＃34;不在关系代数中的SQL语句。

我的查询是这样做的：

从员工中选择员工ID，姓氏和training_id（来自培训），他们接受过本季度培训，但没有完成此培训的所有先决条件 - 因此他们没有完成从业资格培训。

如果结果以X结尾，则表示未成功完成。

TABLES：

employee(employee_id, first_name, last_name)
training(training_id, quartal, year, name)
enrollment(employee_id, training_id, quartal, year, result)
prerequisity(training_id, prerequisity_id)

我的SQL查询：

select e.employee_id, lastname, training_id from employee as e NATURAL JOIN enrollment NATURAL JOIN training NATURAL JOIN prerequisity WHERE quartal = 'first' and year = '2016'  and training_id NOT IN (select e.training_id from enrollment as e NATURAL JOIN employee as ee where e.employee_id=ee.employee_id and result not like '%X' and result is not NULL)

这可能是正确的SQL所以现在我要把它转换成关系代数：

PROJECT[employee_id,lastname, training_id](SELECT[quartal='first',year='2016'](employee@enrollment@training@prerequisity)) # here is the problem

如何模拟＆＃34; NOT IN＆＃34; RA中的命令？

Answer 1

我建议您使用Set Difference (−)

设置差异操作的结果是元组，它们存在于一个关系中但不在第二个关系中。

PROJECT[employee_id,lastname, training_id](SELECT[quartal='first',year='2016']
    (employee@enrollment@training@prerequisity))  - (PROJECT training_id ....

将SQL转换为关系代数 - 下一步该怎么做？

1 个答案: