Question

I have this TSQL code that checks if the ‘sadsadsad’ exists and if not inserts it into the table.

if not exists(select id from [ua_subset_composite] where ua = 'sadsadsad')
  begin
    insert into [ua_subset_composite]
    select 'sadsadsad',1,null,null,null,null
  end

My concern is that in production where there will be multiple threads running concurrently, a situation may occur that a record will slip thru between the not exists select and the insert.

I don’t want to add a unique constraint on the column and wondering if I can improve this SQL code so that it will guarantee the uniqueness

Answer 1

This is what I have ended up doing

insert into [ua_subset_composite]  WITH (TABLOCKX) (ua, os)
select @r, 1 
where not exists (select 1 from [ua_subset_composite] nolock where ua = @r

To test the code I ran this code concurrently from multiple windows

declare @r nvarchar(30);
while(1=1)
begin

set @r =  convert(nvarchar(30),getdate(),21 )
insert into [ua_subset_test]  WITH (TABLOCKX) (ua, os)
select @r, 1 
where not exists (select 1 from [ua_subset_test] nolock where ua = @r

)
end