Constraint with multiple tables. UDF? - sql-server

Question

I have two tables: table A and table B. These two tables are linked with a primary key in A and a foreign key in B.
Table A:
CREATE TABLE [BIO].[table_A](
[table_A_id] [int] IDENTITY(1,1) NOT NULL,
[type_id] [nvarchar](2) NOT NULL
CONSTRAINT [PK_table_A] PRIMARY KEY CLUSTERED
(
[table_A_id] ASC
))
Table B:
CREATE TABLE [BIO].[table_B](
[table_B_id] [int] IDENTITY(1,1) NOT NULL,
[table_A_id] [int] NOT NULL,
[analysis_id] [tinyint] NOT NULL
CONSTRAINT [PK_table_B] PRIMARY KEY CLUSTERED
(
[table_B_id] ASC
))
ALTER TABLE [BIO].[table_B] WITH CHECK
ADD CONSTRAINT [FK_table_B_table_A] FOREIGN KEY([table_A_id])
REFERENCES [BIO].[table_A] ([table_A_id])
GO
ALTER TABLE [BIO].[table_B] CHECK CONSTRAINT [FK_table_B_table_A]
GO
Table B must contain only specific values according to the values in table A.
For example, if I have BL in table A, I can only have 1 or 3 in table B; if I have ST in table A, I can only have 2 or 4 in table B.
I have setup a bridge table which defines these combinations: BL→1 or 3, ST→2 or 4.
Bridge table:
CREATE TABLE [QRY].[bridge_table](
[type_id] [nvarchar](2) NOT NULL,
[analysis_id] [tinyint] NOT NULL,
CONSTRAINT [PK_bridge_table] PRIMARY KEY CLUSTERED
(
[type_id] ASC,
[analysis_id] ASC
))
I am currently using one constraint per table to be sure that any insert or update is correct according to the combinations defined in the bridge table. These two constraints are based on UDFs.
Constraint on table B:
ALTER TABLE [BIO].[Table_B] WITH CHECK
ADD CONSTRAINT [CK_chkAnalysisType]
CHECK (([QRY].[TypeAnalysisMatch_table_B]([table_A_id])>(0)))
GO
UDF:
CREATE FUNCTION [QRY].[TypeAnalysisMatch_table_B] (#table_A_id int)
RETURNS int
AS
BEGIN
RETURN
(
SELECT
Count(BIO.table_A.table_A_id) AS cnt_rec
FROM
QRY.bridge_table
INNER JOIN BIO.table_A ON QRY.bridge_table.type_id = BIO.table_A.type_id
INNER JOIN BIO.table_B ON
QRY.bridge_table.analysis_id = BIO.table_B.analysis_id
AND BIO.table_A.table_A_id = BIO.table_B.table_A_id
WHERE
BIO.table_A.table_A_id = #table_A_id
)
END
It works well for the INSERT but not consistently for the UPDATE. Moreover, as I read that UDFs in constraints should be avoided, I am looking for a better solution.
What would be an efficient alternative to these constraints?

Answer 1

You are right, having UDF in CHECK constraints can be tricky and some UPDATE statements may bypass the check:
http://sqlblog.com/blogs/tibor_karaszi/archive/2009/12/17/be-careful-with-constraints-calling-udfs.aspx
MSSQL: Update statement avoiding the CHECK constraint
As you can see in that SO question the answer recommended to use trigger for the check. Writing a correct efficient trigger is also not an easy task.
I assume that your bridge_table contains this data:
type_id analysis_id
BL 1
BL 3
ST 2
ST 4
I would set up these constraints using only foreign keys, without UDF. It would require some (minimal) duplication of the data, though. I assume that real table_A and table_B have more columns than this example.
1. In table_A include type_id in the primary key:
CREATE TABLE [table_A](
[table_A_id] [int] IDENTITY(1,1) NOT NULL,
[type_id] [nvarchar](2) NOT NULL,
CONSTRAINT [PK_table_A] PRIMARY KEY CLUSTERED
(
[table_A_id] ASC,
[type_id] ASC
))
2. Add a column type_id to the table_B. Yes, the same column as you already have in table_A. This is that duplication of data that I mentioned above:
CREATE TABLE [table_B](
[table_B_id] [int] IDENTITY(1,1) NOT NULL,
[table_A_id] [int] NOT NULL,
[type_id] [nvarchar](2) NOT NULL,
[analysis_id] [tinyint] NOT NULL
CONSTRAINT [PK_table_B] PRIMARY KEY CLUSTERED
(
[table_B_id] ASC
))
3. Make foreign key that links table_B with table_A on two columns (table_A_id, type_id):
ALTER TABLE [table_B] WITH CHECK
ADD CONSTRAINT [FK_table_B_table_A] FOREIGN KEY([table_A_id], [type_id])
REFERENCES [table_A] ([table_A_id], [type_id])
This constraint guarantees that type_id values that are duplicated in table_B will be consistent with original values from table_A.
4. Make foreign key that links table_B with bridge_table again on two columns (type_id, analysis_id):
ALTER TABLE [table_B] WITH CHECK
ADD CONSTRAINT [FK_table_B_bridge_table] FOREIGN KEY([type_id], [analysis_id])
REFERENCES [bridge_table] ([type_id], [analysis_id])
5. Now you can test that everything works as intended.
Add few rows to table_A:
INSERT INTO [table_A] ([type_id])
VALUES ('BL'), ('BL'), ('ST'), ('ZZ');
table_A_id type_id
1 BL
2 BL
3 ST
4 ZZ
Try to insert valid data into table_B:
INSERT INTO [dbo].[table_B] ([table_A_id],[type_id],[analysis_id])
VALUES (1,'BL',1)
INSERT INTO [dbo].[table_B] ([table_A_id],[type_id],[analysis_id])
VALUES (1,'BL',3)
INSERT INTO [dbo].[table_B] ([table_A_id],[type_id],[analysis_id])
VALUES (2,'BL',3)
INSERT INTO [dbo].[table_B] ([table_A_id],[type_id],[analysis_id])
VALUES (3,'ST',2)
INSERT INTO [dbo].[table_B] ([table_A_id],[type_id],[analysis_id])
VALUES (3,'ST',2)
table_B_id table_A_id type_id analysis_id
1 1 BL 1
2 1 BL 3
3 2 BL 3
4 3 ST 2
5 3 ST 2
Try to insert invalid data:
INSERT INTO [dbo].[table_B] ([table_A_id],[type_id],[analysis_id])
VALUES (3,'ST',1)
The INSERT statement conflicted with the FOREIGN KEY constraint "FK_table_B_bridge_table".
The conflict occurred in database "tempdb", table "dbo.bridge_table".
The statement has been terminated.
Which means, that ST can't have analysis_id=1
INSERT INTO [dbo].[table_B] ([table_A_id],[type_id],[analysis_id])
VALUES (3,'BL',1)
The INSERT statement conflicted with the FOREIGN KEY constraint "FK_table_B_table_A".
The conflict occurred in database "tempdb", table "dbo.table_A".
The statement has been terminated.
Which means, that the row in table_A with table_A_id=3 doesn't have BL in type_id.
Foreign keys would keep checking data consistency for all UPDATE statements as well:
UPDATE [dbo].[table_B]
SET [type_id] = 'ST'
WHERE [table_B_id] = 1
The UPDATE statement conflicted with the FOREIGN KEY constraint "FK_table_B_table_A".
The conflict occurred in database "tempdb", table "dbo.table_A".
The statement has been terminated.
UPDATE [dbo].[table_B]
SET [analysis_id] = 2
WHERE [table_B_id] = 1
The UPDATE statement conflicted with the FOREIGN KEY constraint "FK_table_B_bridge_table".
The conflict occurred in database "tempdb", table "dbo.bridge_table".
The statement has been terminated.
But this one works:
UPDATE [dbo].[table_B]
SET [analysis_id] = 3
WHERE [table_B_id] = 1
(1 row(s) affected)