Sql – TSQL foreign keys on views

foreign-keyssqlsql servertsqlviews

I have a SQL-Server 2008 database and a schema which uses foreign key constraints to enforce referential integrity. Works as intended. Now the user creates views on the original tables to work on subsets of the data only. My problem is that filtering certain datasets in some tables but not in others will violate the foreign key constraints.
Imagine two tables "one" and "two". "one" contains just an id column with values 1,2,3. "Two" references "one". Now you create views on both tables. The view for table "two" doesn't filter anything while the view for table "one" removes all rows but the first. You'll end up with entries in the second view that point nowhere.

Is there any way to avoid this? Can you have foreign key constraints between views?

Some Clarification in response to some of the comments:
I'm aware that the underlying constraints will ensure integrity of the data even when inserting through the views. My problem lies with the statements consuming the views. Those statements have been written with the original tables in mind and assume certain joins cannot fail. This assumption is always valid when working with the tables – but views potentially break it.
Joining/checking all constraints when creating the views in the first place is annyoing because of the large number of referencing tables. Thus I was hoping to avoid that.

Best Answer

I love your question. It screams of familiarity with the Query Optimizer, and how it can see that some joins are redundant if they serve no purpose, or if it can simplify something knowing that there is at most one hit on the other side of a join.

So, the big question is around whether you can make a FK against the CIX of an Indexed View. And the answer is no.

create table dbo.testtable (id int identity(1,1) primary key, val int not null);
go
create view dbo.testview with schemabinding as
select id, val
from dbo.testtable
where val >= 50
;
go
insert dbo.testtable
select 20 union all
select 30 union all
select 40 union all
select 50 union all
select 60 union all
select 70 
go
create unique clustered index ixV on dbo.testview(id);
go
create table dbo.secondtable (id int references dbo.testview(id));
go

All this works except for the last statement, which errors with:

Msg 1768, Level 16, State 0, Line 1
Foreign key 'FK__secondtable__id__6A325CF7' references object 'dbo.testview' which is not a user table.

So the Foreign key must reference a user table.

But... the next question is about whether you could reference a unique index that is filtered in SQL 2008, to achieve a view-like FK.

And still the answer is no.

create unique index ixUV on dbo.testtable(val) where val >= 50;
go

This succeeded.

But now if I try to create a table that references the val column

create table dbo.thirdtable (id int identity(1,1) primary key, val int not null check (val >= 50) references dbo.testtable(val));

(I was hoping that the check constraint that matched the filter in the filtered index might help the system understand that the FK should hold)

But I get an error saying:

There are no primary or candidate keys in the referenced table 'dbo.testtable' that matching the referencing column list in the foreign key 'FK__thirdtable__val__0EA330E9'.

If I drop the filtered index and create a non-filtered unique non-clustered index, then I can create dbo.thirdtable without any problems.

So I'm afraid the answer still seems to be No.

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