SQL set check constraint - sql

Question

I have a problem with setting check constrain. I have table Policy where primary key is set on (Policy_id, History_id) + additional columns and table Report which have Policy_id and some additional columns.
How can I set check constraint statement on Report table to check if policy_id exists in Policy table?
I cannot use foreign key constrain because Report do not have history_id column
Report cannot contain record with Policy_id if it do not exists in Policy table and hence, cannot perform insert into Report

Answer 1

If the Policy_id and History_id are a composite primary key then the foreign key on the referencing table must also hold the both columns.
If you really need to check just for one of them (the policy_id) I guess you'd have to do it manually which is not a good idea.
It would be better if the Report table would have 2 foreign keys and both the policy_id and history_id would be a single primary keys.

Answer 2

You could create a separate table just for the purposes of this foreign key constraint and then use triggers to maintain this data:
CREATE TABLE ExistingPolicies (
PolicyID int not null,
PolicyCount int not null,
constraint PK_ExistingPolicies PRIMARY KEY (PolicyID)
)
And then the triggers:
CREATE TRIGGER T_Policy_I
on Policy
instead of insert
as
;With totals as (
select PolicyID,COUNT(*) as cnt from inserted
group by PolicyID
)
merge into ExistingPolicies t
using totals s
on
t.PolicyID = s.PolicyID
when matched then update set PolicyCount = PolicyCount + s.cnt
when not matched then insert (PolicyID,PolicyCount) values (s.PolicyID,s.cnt);
go
CREATE TRIGGER T_Policy_D
on Policy
instead of delete
as
;With totals as (
select PolicyID,COUNT(*) as cnt from deleted
group by PolicyID
)
merge into ExistingPolicies t
using totals s
on
t.PolicyID = s.PolicyID
when matched and t.PolicyCount = s.cnt then delete
when matched then update set PolicyCount = PolicyCount - s.cnt;
go
CREATE TRIGGER T_Policy_U
on Policy
instead of update
as
;With totals as (
select PolicyID,SUM(cnt) as cnt
from
(select PolicyID,1 as cnt from inserted
union all
select PolicyID,-1 as cnt from deleted
) t
group by PolicyID
)
merge into ExistingPolicies t
using totals s
on
t.PolicyID = s.PolicyID
when matched and t.PolicyCount = -s.cnt then delete
when matched then update set PolicyCount = PolicyCount + s.cnt
when not matched then insert (PolicyID,PolicyCount) values (s.PolicyID,s.cnt);
go
(Code not tested but should be close to correct)

Answer 3

I think using a check constraint is a fine idea here.
Write a function that accepts Policy_id as a parameter, and does a query to check if the policy exists in the Policy table, and returns a simple 1 (exists) or 0 (does not exist).
Then set your check constraint on your Report Table to dbo.MyFunction(Policy_Id)=1
That's it.