Question

we are trying to create a constraint that decides whether a grade should be F or not. If a course isn’t finished then the student should get the grade F. Else the student should get another grade.

This is our constraint

ALTER TABLE Registrerad_på ADD CONSTRAINT chk_avslutadkurs CHECK (CASE WHEN FinishedCourse 
= 'No' THEN Grade = 'F')

Grade is a varchar(50) column that accepts null values and FinishedCourse is the same, except it cant contain null values and it only contains ‘Yes’ values at this time. The table Registrerad_på contains data (int and varchar(50)).

The question: What is wrong with the Check, the error arises at the last “=” at the THEN statement. Please tell me if i need to add some more information, sorry if it is unclear.

Any hint would be appreciated.

Answer 1

Your constraint is

ALTER TABLE Registrerad_på 
ADD CONSTRAINT chk_avslutadkurs 
CHECK (CASE WHEN FinishedCourse = 'No' THEN Grade = 'F')

The skeleton syntax for a CASE statement (standard SQL) is usually

CASE WHEN ... THEN ... ELSE ... END 

So you need at least the END keyword.

CHECK (CASE WHEN FinishedCourse = 'No' THEN Grade = 'F' END)

But you probably really need something more like this.

CHECK (CASE WHEN FinishedCourse = 'No' THEN Grade = 'F' 
            WHEN FinishedCourse = 'Yes' THEN Grade IN ('A','B','C','D','F')
            ELSE Grade = 'Somebody let some bad data into FinishedCourse'
       END)

Think about adding another constraint on FinishedCourse.

CHECK (FinishedCourse IN ('Yes', 'No') )