Enforcing Data Integrity Across Three Columns in SQL

I have a bit of a question regarding data integrity on a table I'm creating. For reference, here is the code to create the table:

CREATE TABLE tbl_Course_ExamModules
		CourseExamModuleID INT IDENTITY(1,1) NOT NULL,
		CourseCode VARCHAR(3) NOT NULL,
		ExamModule VARCHAR(10) NOT NULL,
		CashInCode BIT NULL,
		AcademicYear INT NOT NULL,
		StudentLevel INT NOT NULL,
		CourseModuleCount INT NOT NULL
ALTER TABLE tbl_Course_ExamModules
	ADD CONSTRAINT pk_Course_ExamModules_CourseExamModuleID 

CREATE UNIQUE NONCLUSTERED INDEX idx_CourseExamModules_Code_Module_Year
	ON tbl_Course_ExamModules (CourseCode, ExamModule, AcademicYear)

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Which is all fine and fairly straight forward. Each module can only be associated with each course once for each academic year.
Then I start to struggle. For each course there is a module which is called the "Cash-In Code" Each course can only have one cash-in code for each Student Level. There can be multiple modules which are not cash-in codes for each course within an academic year and student level. So, I can't place a unique index on the CashInCode, CourseCode, AcademicYear, StudentLevel columns the CashInCode column can have the value 0 more than once.
I hope that makes sense!
I looked at adding a WITH CHECK Constraint, but I don't think they can be applied the way I want (or can they?). Is there a solution to this without using a trigger?
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Barry CunneyConnect With a Mentor Commented:
Hi Karl_Mark,
As a follow on from my previous post, something like the below may be an option:

CREATE FUNCTION CheckThatCourseHasOnlyOneCashInCode(@Course_code varchar(3), @StudentLevel int)
    DECLARE @count int
    SELECT @count = COUNT(*) FROM dbo.tbl_Course_ExamModules WHERE CourseCode = @course_code AND StudentLevel = @StudentLevel AND CashInCode = 1
    RETURN @count <= 1

ALTER TABLE dbo.tbl_Course_ExamModules CONSTRAINT OneCashInCodePerCourse CHECK (dbo.CheckThatCourseHasOnlyOneCashInCode(CourseCode, StudentLevel));

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Barry CunneyCommented:
Hi Karl_Mark,
It is possible to add a constraint which references a function - e.g.
CONSTRAINT chkCheckAge CHECK(dbo.fnGetAge(DateOfBirth) >= 18)

so the use of a function in a CHECK constraint may open up some possibilities for your scenario
Karl_markAuthor Commented:
Thanks Barry. Very close to the solution I came up with. I hadn't thought of using a function to do the job!
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