How can this SQL check constraint for time ranges fail, when table is empty? - sql

Question

I have implemented a time range validation, as a check constraint, using a function in SQL, using this guide, almost to the letter.
Creating the function first:
create function dbo.ValidateStatusPeriodInfoTimeRange
(
#btf_id VARCHAR(32),
#start_time BIGINT,
#end_time BIGINT
)
returns bit
as
begin
declare #Valid bit = 1;
if exists( select *
from dbo.StatusPeriodInfoOccurrence o
where o.btf_id = #btf_id
and #start_time <= o.end_time and o.start_time <= #end_time )
set #Valid = 0;
return #Valid;
end
And then the constraint, using the function:
alter table dbo.StatusPeriodInfoOccurrence with nocheck add constraint
CK_StatusPeriodInfoOccurrence_ValidateTimeRange
check (dbo.ValidateStatusPeriodInfoTimeRange(btf_id, start_time, end_time) = 1);
When I try to insert an element into a completely empty table, I get:
The INSERT statement conflicted with the CHECK constraint
"CK_StatusPeriodInfoOccurrence_ValidateTimeRange". The conflict occurred in
database "D600600TD01_BSM_Surveillance", table "dbo.StatusPeriodInfoOccurrence".
I tried to figure out if I did something wrong in the function itself, and created this query to check it's return value:
DECLARE #ReturnValue INT
EXEC #ReturnValue = ValidateStatusPeriodInfoTimeRange
#btf_id = 'a596933eff9143bceda5fc5d269827cd',
#start_time = 2432432,
#end_time = 432432423
SELECT #ReturnValue
But this returns 1, as it should.
I am at a loss on how to continue debugging this. All parts seem to work, but the whole does not. Any ideas on how the insert statement can conflict with the check constraint?
Edit: Here is my insert statement for completion:
INSERT INTO StatusPeriodInfoOccurrence (btf_id, start_time, end_time) VALUES ('a596933eff9143bceda5fc5d269827cd',2432432,432432423);
There is an additional primary key comlumn with identity auto increment.

Answer 1

CHECK constraints happen after the row is inserted, so in its current form, the constraint fails because the very row that was inserted matches the constraint. In order for this to work as a constraint (not a trigger) there must be a way to distinguish the row we're checking from all other rows. Michał's answer shows how to do this without relying on an IDENTITY, but if you do have that explicitly excluding the row may be simpler:
create function dbo.ValidateStatusPeriodInfoTimeRange
(
#id INT,
#btf_id VARCHAR(32),
#start_time BIGINT,
#end_time BIGINT
)
returns bit
as
begin
declare #Valid bit = 1;
if exists( select *
from dbo.StatusPeriodInfoOccurrence o
where o.id <> #id AND o.btf_id = #btf_id
and #start_time <= o.end_time and o.start_time <= #end_time )
set #Valid = 0;
return #Valid;
end;
with the constraint defined as
check (dbo.ValidateStatusPeriodInfoTimeRange(id, btf_id, start_time, end_time) = 1)
Regardless of the approach, indexes on (btf_id, start_time) and (btf_id, end_time) are a good idea to keep this scalable, otherwise a full table scan is necessary on every insert.

Answer 2

As was mentioned in comments, constraint is checked after the record is inserted into a table, then the transaction is commited or rolled back, depending on result of a check, which in your example will always fails, as query:
select *
from dbo.StatusPeriodInfoOccurrence o
where o.btf_id = #btf_id
and #start_time <= o.end_time and o.start_time <= #end_time
will return always at least one row (the one being inserted).
So, knowing that, you should check if the query returns more than one record, so the condition in if statement should become:
if (select count(*)
from dbo.StatusPeriodInfoOccurrence o
where o.btf_id = #btf_id
and #start_time <= o.end_time and o.start_time <= #end_time ) > 1
This solution works fine (tested on my DB).