External key relationship with condition
Good morning,
I have a main table with id and type. Depending on the type, I have tables for children that use this id as a foreign key to ensure consistency. For example. for the main table:
master_ID, type
11, A
12, B
13, A
For a child table named Child_A that stores additional data for type A;
Child_A_ID, FK_master_ID, .... 1, 11, .... 2, 13, ....
How can I prevent the type in my main table from changing to a different value when there is a matching record in my child table. My referential integrity is currently preserved, but it makes no sense to store in the information Child_A of type A, then the record in the main table is of a different type.
change:
Will the only key be a foreign key with two attributes (id and type) and a duplicate type in each child table? For example. for table child_A;
Child_A_ID, FK_master_ID, type, ....
1, 11, A, ....
2, 13, A, ....
Hope this is clear enough.
Sylvain
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You can create a check constraint that uses a custom function to determine if an identifier value is contained in a matching type table.
ALTER TABLE MasterTable
ADD CONSTRAINT CHK_MasterTable_Type
CHECK(dbo.fn_check_IdBelongsToType(master_ID, type) = 1)
and in the function itself, you do something like this:
CREATE FUNCTION fn_check_IdBelongsToType (
@master_ID int,
@type char(1)
)
RETURNS int
AS
BEGIN
IF @Type = 'A' AND EXISTS (
SELECT 1
FROM Child_A
WHERE FK_master_ID = @master_ID
) RETURN 1
IF @Type = 'B' AND EXISTS (
SELECT 1
FROM Child_B
WHERE FK_master_ID = @master_ID
) RETURN 1
IF @Type = 'C' AND EXISTS (
SELECT 1
FROM Child_C
WHERE FK_master_ID = @master_ID
) RETURN 1
-- after testing all child tables, return 0 to indicate that the value was not found
RETURN 0
END
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