外键上的SQL约束

Question

I am having problems creating a constraint for a foreign key field, so that a foreign key can only be entered if that foreign key links to a row containing a specific attribute.

To give a better idea about what I mean, I have created the following example. The Employee entity table contains a Grade field, which can either be 'S' for a senior level employee, or 'J' for a junior level employee. In the Expenses table, I want to limit any entries into the ApprovedBy field to those EmpNo values that have a Grade field containing 'S' in the Employee table.

CREATE TABLE Employee
(EmpNo INT PRIMARY KEY,
FirstName VARCHAR(15),
LastName VARCHAR(15),
Grade CHAR(1),
CONSTRAINT chk_ValidGrade CHECK (Grade IN ('J','S'))
)

CREATE TABLE Expenses
(ExpenseId INT IDENTITY(1,1) PRIMARY KEY,
Amount FLOAT,
ApprovedBy INT FOREIGN KEY REFERENCES Employee(EmpNo),
)

I instinctively want to use a join, or other relational algebra function to do this. However, my understanding is that you can't use the SELECT function within a CHECK function, so I'm not sure how I would define the constraint.

Answer 1

As far as I know, you cannot use check constraint for this task. You have to use a trigger for insert and update in your Expenses table.