How to create a column null or not-null dependent on the value of another column? - sql-server

Question

I'm using database first approach with EF core and trying to figure out a clean solution to the below problem -
Consider a Student attendance table (irrelevant columns removed) below that stores date of class and allows the student to enter his class rating -
create table Student (
Id int Identity(1, 1) not null,
ClassDate smalldatetime not null,
ClassRatingByStudent varchar(250) not null
)
This is a webapp where school attendance system automatically populates the above table at EOD and then the student (let's say a few days later) is required to add class ratings. When the table is populated by the school attendance system, there is nothing in the ClassRatingByStudent column. Then when the student logs in, he must add the rating.
As you see, ClassRatingByStudent must be null when the school attendance system populates the table and must be not-null when the student saves his changes. One obvious solution is make ClassRatingByStudent column nullable ad handle it in the code but I'm wondering if there is a neater database (or maybe EF) level solution exists or some sort of pattern/architecture guidelines for this type of scenarios?

Answer 1

I don't know but maybe CHECK constraint could help you:
CREATE TABLE TestTable(
ID int NOT NULL IDENTITY,
RatingAllowed bit NOT NULL DEFAULT 0, -- switcher
RatingValue varchar(250),
CONSTRAINT PK_TestTable PRIMARY KEY(ID),
CONSTRAINT CK_TestTable_RatingValue CHECK( -- constraint
CASE
WHEN RatingAllowed=0 AND RatingValue IS NULL THEN 1
WHEN RatingAllowed=1 AND RatingValue IS NOT NULL THEN 1
ELSE 0
END=1
)
)
INSERT TestTable(RatingAllowed,RatingValue)VALUES(0,NULL)
INSERT TestTable(RatingAllowed,RatingValue)VALUES(1,'AAA')
-- The INSERT statement conflicted with the CHECK constraint "CK_TestTable_RatingValue"
INSERT TestTable(RatingAllowed,RatingValue)VALUES(0,'AAA')
INSERT TestTable(RatingAllowed,RatingValue)VALUES(1,NULL)

Answer 2

I found a variant how to check using another table as switcher
CREATE TABLE TableA(
ID int NOT NULL IDENTITY PRIMARY KEY,
StudentID int NOT NULL,
Grade int
)
CREATE TABLE TableB(
StudentID int NOT NULL PRIMARY KEY
)
GO
-- auxiliary function
CREATE FUNCTION GradeIsAllowed(#StudentID int)
RETURNS bit
BEGIN
DECLARE #Result bit=CASE WHEN EXISTS(SELECT * FROM TableB WHERE StudentID=#StudentID) THEN 1 ELSE 0 END
RETURN #Result
END
GO
-- constraint to check
ALTER TABLE TableA ADD CONSTRAINT CK_TableA_Grade CHECK(
CASE dbo.GradeIsAllowed(StudentID) -- then we can use the function here
WHEN 1 THEN CASE WHEN Grade IS NOT NULL THEN 1 ELSE 0 END
WHEN 0 THEN CASE WHEN Grade IS NULL THEN 1 ELSE 0 END
END=1)
GO
-- Tests
INSERT TableB(StudentID)VALUES(2) -- allowed student
INSERT TableA(StudentID,Grade)VALUES(1,NULL) -- OK
INSERT TableA(StudentID,Grade)VALUES(2,5) -- OK
INSERT TableA(StudentID,Grade)VALUES(1,4) -- Error
INSERT TableA(StudentID,Grade)VALUES(2,NULL) -- Error
INSERT TableB(StudentID)VALUES(1) -- add 1
UPDATE TableA SET Grade=4 WHERE StudentID=1 -- OK
UPDATE TableA SET Grade=NULL WHERE StudentID=1 -- Error