引用同一个主键的两个外键

Question

Is this okay to have two foreign keys in one table referencing one primary key of other table?

EmployeeID is a primary key in the employee table and appearing as a foreign key twice in the timesheet table.

There will be few admin users filling up timsheets on the behalf of other employees.

In the timsheet table field 'TimsheetFor' will have employeeID of that person who has worked on projects and field 'EnteredBy' or 'FilledBy' will have employeeid of that person who has filled up this timesheet.

Which of the following option is correct?

NOTE: Tables are showing only those fields which are related to this question.

Answer 1

I would go with option 1 . It is perfectly fine to have two foreign key columns referencing the same primary key column in a different table since each foreign key value will reference a different record in the related table.

I'm sure option 2 would work, but you would essentially have a 1-to-1 relationship between TIMESHEET_TABLE and TIMESHEET_FILLED_BY , making two tables unnecessary and more difficult to maintain.

In fact, if both ENTERED_BY and TIMESHEET_FOR are required in pairs, using option 1 makes far more sense because this is automatically enforced by the database and foreign keys.

Answer 2

Option 1 is a perfect solution. You may define foreign key constraint as following

1st foreign key constraint for Timesheet_For column

ALTER TABLE TIMESHEETTABLE 
ADD CONSTRAINT fk_TimesheetTable_EmployeeTable
FOREIGN KEY (TIMESHEET_FOR)
REFERENCES EMPLOYEETABLE(EMPLOYEE_ID)

2nd foreign key constraint for Entered_By column

ALTER TABLE TIMESHEETTABLE 
ADD CONSTRAINT fk_TimesheetTable_EmployeeTable_1
FOREIGN KEY (ENTERED_BY)
REFERENCES EMPLOYEETABLE(EMPLOYEE_ID)

Answer 3

是的，这没有问题……您可以将另一个表中一个表的主键用作外键两次。

Answer 4

Your query like :

SELECT t.EMPLOYEE_ID, a.NAME as TimeSheetFor, b.NAME as EnteredBy 
FROM timesheet t
JOIN employee a ON t.timesheet_for =a.employee_id
JOIN employee b ON t.entered_by = b.employee_id

Using this query you will get result as you want.

Answer 5

我有一个相同的问题，因为当我将更新规则设置为（级联）而不是（没有动作）来删除规则时，我收到一条错误消息实际上是在尝试在itemUnit2ID与表的第二个外键之间建立关系时发生的[项目]和来自表格[units]的对应主键[unitId]