Values controlled by foreign keys

Question

I have very simple database in SQL Server, with these three tables:

• Theater (ID, is3D, othervalues...)
• Show (

Answer 1

To do this through foreign keys alone you need to add an is3D column to Show as well and a couple of logically redundant UNIQUE constraints.

CREATE TABLE Theater
  (
     ID   INT PRIMARY KEY,
     is3D BIT NOT NULL,
     /*Other columns*/
     UNIQUE(ID, is3D)
  )

CREATE TABLE Movie
  (
     ID   INT PRIMARY KEY,
     is3D BIT NOT NULL,
     /*Other columns*/
     UNIQUE(ID, is3D)
  )

CREATE TABLE Show
  (
     ID         INT PRIMARY KEY,
     Theater_ID INT NOT NULL,
     Movie_ID   INT NOT NULL,
     is3D       BIT NOT NULL,
     /*Other columns*/
     FOREIGN KEY(Theater_ID, is3D) REFERENCES Theater (ID, is3D),
     FOREIGN KEY(Movie_ID, is3D) REFERENCES Movie (ID, is3D)
  ) 

An indexed view can also be used to enforce this declaratively without requiring the additional column or unique constraints as below.

CREATE TABLE dbo.TwoRows
  (
     X INT PRIMARY KEY
  );

INSERT INTO dbo.TwoRows
VALUES      (1), (2)

GO

CREATE VIEW V
WITH SCHEMABINDING
AS
  SELECT S.Theater_ID,
         S.Movie_ID
  FROM   dbo.Show S
         JOIN dbo.Theater T
           ON S.Theater_ID = T.ID
         JOIN dbo.Movie M
           ON S.Movie_ID = M.ID
         CROSS JOIN dbo.TwoRows
  WHERE  T.is3D <> M.is3D

GO

CREATE UNIQUE CLUSTERED INDEX IX
  ON V(Theater_ID, Movie_ID) 

The underlying query should always return no rows if the rule is satisfied. If any row is returned then the cross join on dbo.TwoRows will multiply it out causing a unique constraint violation and preventing the situation.