Is it possible to add a logic Constraint to a Foreign Key?

Question

I\'ve got two tables and I\'ve added a foreign key constraint. Kewl - works great. Now, is it possible to further constrain that relationship against some data in the parent

Answer 1

Here is an alternative approach that builds on the technique discussed in "Enforcing Complex Constraints with Indexed Views" to enforce the AnimalType constraint. The advantage of this approach is that it avoids the extra column in the subtype table; however, it is slower than the composite foreign key presented in AlexKuznetsov's answer (although nowhere near as abysmally slow as the UDF approach).

I ran the same series of tests as AlexKuznetsov, but increased the number of rows from 131,072 to 1,048,576. Here is a summary of the results:

-- No integrity check:                         2488 ms.
-- With composite foreign key:                 4404 ms.
-- With indexed view and unique index:         7063 ms. <- new
-- With check constraint calling scalar UDF:  78304 ms.

Schema definitions:

CREATE TABLE dbo.Mammals4
(
    AnimalId INT NOT NULL PRIMARY KEY
        REFERENCES dbo.Animals (AnimalId),
    SomeOtherStuff VARCHAR(10)
);

CREATE TABLE dbo.TwoRows   -- can be reused for other similar indexed views
(
    n TINYINT NOT NULL PRIMARY KEY
)

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

GO

CREATE VIEW dbo.CK_Mammals4 WITH SCHEMABINDING
AS
    SELECT M.AnimalId
    FROM dbo.Mammals4 M
        JOIN dbo.Animals A
            ON A.AnimalId = M.AnimalId
        CROSS JOIN dbo.TwoRows
    WHERE A.AnimalType != 1

GO

CREATE UNIQUE CLUSTERED INDEX CKIX_Mammals4
ON dbo.CK_Mammals4 (AnimalId)

Tests and results:

-- No integrity check:

INSERT INTO dbo.Mammals3 (AnimalId, SomeOtherStuff)
SELECT n, 'some info' FROM dbo.Numbers;

Table 'Mammals3'. Scan count 0, logical reads 2969171, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1724, physical reads 3, read-ahead reads 1720, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 2247 ms,  elapsed time = 2488 ms.

-- With composite foreign key:

INSERT INTO dbo.Mammals2 (AnimalId, AnimalType, SomeOtherStuff)
SELECT n, 1, 'some info' FROM dbo.Numbers;

Table 'Animals'. Scan count 1, logical reads 1432, physical reads 3, read-ahead reads 1428, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Mammals2'. Scan count 0, logical reads 3163291, physical reads 3, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1724, physical reads 3, read-ahead reads 1720, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 3822 ms,  elapsed time = 4404 ms.

-- With indexed view and unique index:

INSERT INTO dbo.Mammals4 (AnimalId, SomeOtherStuff)
SELECT n, 'some info' FROM dbo.Numbers

Table 'CK_Mammals4'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Animals'. Scan count 2, logical reads 4953, physical reads 6, read-ahead reads 4945, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Worktable'. Scan count 2, logical reads 2978842, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Mammals4'. Scan count 0, logical reads 3162616, physical reads 3, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1724, physical reads 3, read-ahead reads 1720, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 6209 ms,  elapsed time = 7063 ms.

-- With check constraint calling scalar UDF:

INSERT INTO dbo.Mammals (AnimalId, SomeOtherStuff)
SELECT n, 'some info' FROM dbo.Numbers;

Table 'Mammals'. Scan count 0, logical reads 2969171, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Numbers'. Scan count 1, logical reads 1724, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 77424 ms,  elapsed time = 78304 ms.