SQL Server – Resolve Foreign Key Constraint Violation Issue

I have identified 3 situations.

1. A student with no enrollments.
2. A student with enrollments but no grades.
3. A student with enrollments and grades.

There is a trigger on the enrollments table to calculate GPA. If a student has grades it will update or insert an entry into the GPA table; no grades, no GPA table entry.

I can delete a student with no enrollments (#1). I can delete a student with enrollments and grades (#3 above). But I can not delete a student with enrollments but no grades (#2). I get a reference constraint violation.

The DELETE statement conflicted with the REFERENCE constraint "FK_dbo.GPA_dbo.Student_StudentID". The conflict occurred in database "", table "dbo.GPA", column 'StudentID'.

If I could not delete a new student with no enrollments (and no GPA entry) then I would understand the constraint violation, but I can delete that student. It's a student with enrollments and no grades (and still no GPA entry) that I can not delete.

I have patched my trigger so I can go forward. Now, if you have enrollments the trigger inserts you into the GPA table no matter what. But I don't understand the underlying problem. Any explanation would be most appreciated.

For what it's worth:

1. Visual Studio 2013 Professional.
2. IIS express (internal to VS2013).
3. ASP.NET Web App using EntityFramework 6.1.1.
4. MS SQL Server 2014 Enterprise.
5. GPA.Value is nullable.
6. Enrollment.GradeID is nullable.

Here is a snippet of the database:

—EDIT—

The tables are all created by the EntityFramework, I used SQL Server Management Studio to produce these.

Here are the create table statements with constraints.:

GPA table:

CREATE TABLE [dbo].[GPA](
    [StudentID] [int] NOT NULL,
    [Value] [float] NULL,
  CONSTRAINT [PK_dbo.GPA] PRIMARY KEY CLUSTERED 
  (
    [StudentID] ASC
  )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, 
         ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

ALTER TABLE [dbo].[GPA]  WITH CHECK 
  ADD  CONSTRAINT [FK_dbo.GPA_dbo.Student_StudentID] 
  FOREIGN KEY([StudentID])
  REFERENCES [dbo].[Student] ([ID])

ALTER TABLE [dbo].[GPA] 
  CHECK CONSTRAINT [FK_dbo.GPA_dbo.Student_StudentID]

Enrollment table:

CREATE TABLE [dbo].[Enrollment](
    [EnrollmentID] [int] IDENTITY(1,1) NOT NULL,
    [CourseID] [int] NOT NULL,
    [StudentID] [int] NOT NULL,
    [GradeID] [int] NULL,
  CONSTRAINT [PK_dbo.Enrollment] PRIMARY KEY CLUSTERED 
  (
    [EnrollmentID] ASC
  )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, 
         ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

ALTER TABLE [dbo].[Enrollment]  WITH CHECK 
  ADD  CONSTRAINT [FK_dbo.Enrollment_dbo.Course_CourseID] 
  FOREIGN KEY([CourseID])
  REFERENCES [dbo].[Course] ([CourseID])
  ON DELETE CASCADE

ALTER TABLE [dbo].[Enrollment] 
  CHECK CONSTRAINT [FK_dbo.Enrollment_dbo.Course_CourseID]

ALTER TABLE [dbo].[Enrollment]  WITH CHECK 
  ADD  CONSTRAINT [FK_dbo.Enrollment_dbo.Grade_GradeID] 
  FOREIGN KEY([GradeID])
  REFERENCES [dbo].[Grade] ([GradeID])

ALTER TABLE [dbo].[Enrollment] 
  CHECK CONSTRAINT [FK_dbo.Enrollment_dbo.Grade_GradeID]

ALTER TABLE [dbo].[Enrollment]  WITH CHECK 
  ADD  CONSTRAINT [FK_dbo.Enrollment_dbo.Student_StudentID] 
  FOREIGN KEY([StudentID])
  REFERENCES [dbo].[Student] ([ID])
  ON DELETE CASCADE

ALTER TABLE [dbo].[Enrollment] 
  CHECK CONSTRAINT [FK_dbo.Enrollment_dbo.Student_StudentID]

Student table:

CREATE TABLE [dbo].[Student](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [EnrollmentDate] [datetime] NOT NULL,
    [LastName] [nvarchar](50) NOT NULL,
    [FirstName] [nvarchar](50) NOT NULL,
  CONSTRAINT [PK_dbo.Student] PRIMARY KEY CLUSTERED 
  (
    [ID] ASC
  )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, 
         ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

Here are the triggers:

CREATE TRIGGER UpdateGPAFromUpdateDelete
ON Enrollment
AFTER UPDATE, DELETE AS
BEGIN
    DECLARE @UpdatedStudentID AS int
    SELECT @UpdatedStudentID = StudentID FROM DELETED
    EXEC MergeGPA @UpdatedStudentID
END

CREATE TRIGGER UpdateGPAFromInsert
ON Enrollment
AFTER INSERT AS
--DECLARE @InsertedGradeID AS int
--SELECT @InsertedGradeID = GradeID FROM INSERTED
--IF @InsertedGradeID IS NOT NULL
    BEGIN
        DECLARE @InsertedStudentID AS int
        SELECT @InsertedStudentID = StudentID FROM INSERTED
        EXEC MergeGPA @InsertedStudentID
    END

The patch to move forward was to comment out those lines in the AFTER INSERT trigger.

Here is the stored procedure:

CREATE PROCEDURE MergeGPA @StudentID int AS
MERGE GPA AS TARGET
USING (SELECT @StudentID) as SOURCE (StudentID)
ON (TARGET.StudentID = SOURCE.StudentID)
WHEN MATCHED THEN
    UPDATE
        SET Value = (SELECT Value FROM GetGPA(@StudentID))
WHEN NOT MATCHED THEN
INSERT (StudentID, Value)
    VALUES(SOURCE.StudentID, (SELECT Value FROM GetGPA(@StudentID)));

Here is the database function:

CREATE FUNCTION GetGPA (@StudentID int) 
RETURNS TABLE
AS RETURN
SELECT ROUND(SUM (StudentTotal.TotalCredits) / SUM (StudentTotal.Credits), 2) Value
    FROM (
        SELECT 
            CAST(Credits as float) Credits
            , CAST(SUM(Value * Credits) as float) TotalCredits
        FROM 
            Enrollment e 
            JOIN Course c ON c.CourseID = e.CourseID
            JOIN Grade g  ON e.GradeID = g.GradeID
        WHERE
            e.StudentID = @StudentID AND
            e.GradeID IS NOT NULL
        GROUP BY
            StudentID
            , Value
            , e.courseID
            , Credits
    ) StudentTotal

Here is the debug output from the controller's delete method, the select statement is the method querying what to delete. This student has 3 enrollments, the REFERENCE constraint issue happens when the 3rd enrollment deletes. I presume EF is using a transaction because the enrollments are not deleted.

iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.ReaderExecuted;Timespan:00:00:00.0004945;Properties:
Command: SELECT 
    [Project2].[StudentID] AS [StudentID], 
    [Project2].[ID] AS [ID], 
    [Project2].[EnrollmentDate] AS [EnrollmentDate], 
    [Project2].[LastName] AS [LastName], 
    [Project2].[FirstName] AS [FirstName], 
    [Project2].[Value] AS [Value], 
    [Project2].[C1] AS [C1], 
    [Project2].[EnrollmentID] AS [EnrollmentID], 
    [Project2].[CourseID] AS [CourseID], 
    [Project2].[StudentID1] AS [StudentID1], 
    [Project2].[GradeID] AS [GradeID]
    FROM ( SELECT 
        [Limit1].[ID] AS [ID], 
        [Limit1].[EnrollmentDate] AS [EnrollmentDate], 
        [Limit1].[LastName] AS [LastName], 
        [Limit1].[FirstName] AS [FirstName], 
        [Limit1].[StudentID] AS [StudentID], 
        [Limit1].[Value] AS [Value], 
        [Extent3].[EnrollmentID] AS [EnrollmentID], 
        [Extent3].[CourseID] AS [CourseID], 
        [Extent3].[StudentID] AS [StudentID1], 
        [Extent3].[GradeID] AS [GradeID], 
        CASE WHEN ([Extent3].[EnrollmentID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
        FROM   (SELECT TOP (2) 
            [Extent1].[ID] AS [ID], 
            [Extent1].[EnrollmentDate] AS [EnrollmentDate], 
            [Extent1].[LastName] AS [LastName], 
            [Extent1].[FirstName] AS [FirstName], 
            [Extent2].[StudentID] AS [StudentID], 
            [Extent2].[Value] AS [Value]
            FROM  [dbo].[Student] AS [Extent1]
            LEFT OUTER JOIN [dbo].[GPA] AS [Extent2] ON [Extent1].[ID] = [Extent2].[StudentID]
            WHERE [Extent1].[ID] = @p__linq__0 ) AS [Limit1]
        LEFT OUTER JOIN [dbo].[Enrollment] AS [Extent3] ON [Limit1].[ID] = [Extent3].[StudentID]
    )  AS [Project2]
    ORDER BY [Project2].[StudentID] ASC, [Project2].[ID] ASC, [Project2].[C1] ASC: 
iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.NonQueryExecuted;Timespan:00:00:00.0012696;Properties:
Command: DELETE [dbo].[Enrollment]
WHERE ([EnrollmentID] = @0): 
iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.NonQueryExecuted;Timespan:00:00:00.0002634;Properties:
Command: DELETE [dbo].[Enrollment]
WHERE ([EnrollmentID] = @0): 
iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.NonQueryExecuted;Timespan:00:00:00.0002512;Properties:
Command: DELETE [dbo].[Enrollment]
WHERE ([EnrollmentID] = @0): 
iisexpress.exe Error: 0 : Error executing command: DELETE [dbo].[Student]
WHERE ([ID] = @0) Exception: System.Data.SqlClient.SqlException (0x80131904): The DELETE statement conflicted with the REFERENCE constraint "FK_dbo.GPA_dbo.Student_StudentID". The conflict occurred in database "<databasename>", table "dbo.GPA", column 'StudentID'.
The statement has been terminated.