Question

To elabortate,

is it possible to enforce a rule where ONLY one record entry can have a column named ‘IsPrimaryUser’ set to true, whereas all others grouped by another column are set to false. The condition for deciding which entry will have a true ‘IsPrimaryUser’ field will be the CompanyId column.

I am only interested in whether it can be done using check constraints. Obviously, there is a SQL approach to something like this.

Example:

User Table

int UserId | int CompanyId | bit IsPrimaryUser

Data:

UserId  |  CompanyId  | IsPrimaryUser
   1          1              1
   2          1              0
   3          1              0
   4          1              0
   5          2              1
   6          2              0
   7          2              0
   8          2              0

Answer 1

Check Constraints only work on a single row, but you can use scalar UDFs within the constraint.

You can breach the single-row check by using UDFs that check other rows in the table. Although unlike a trigger where you can access the DELETED virtual table and process individually, SQL Server seems to hold the records in a sort of transaction, and perform a CHECK on EACH row after the change, then finally accepting or aborting the CRUD in batch.

See this test case

Create table

create table usertable (UserId int,
CompanyId int, IsPrimaryUser int)

Populate

insert usertable select
1, 1, 1 union all select
2, 1, 0 union all select
3, 1, 0 union all select
4, 1, 0 union all select
5, 2, 1 union all select
6, 2, 0 union all select
7, 2, 0 union all select
8, 2, 0

Scalar function helper

create function dbo.anyprimaryuser(@userid int, @company int) returns bit as
begin
return
    case when exists (
    select * from usertable
    where companyid=@company and isprimaryuser=1 and userid<>@userid)
    then 1 else 0 end
end

The CHECK constraint

alter table usertable add constraint usertable_ck1
check (isprimaryuser=0 or dbo.anyprimaryuser(userid,companyid)=0)

Tests

insert usertable select 9,2,1 -- fail

insert usertable select 9,2,0 -- ok

insert usertable select 19,4,1 union all select 20,4,0 -- ok

insert usertable select 19,3,1 union all select 20,3,0 union all select 21,3,1
-- not ok, accepting the multi-row insert will breach the constraint

update usertable set IsPrimaryUser=1-IsPrimaryUser where CompanyId=4
-- ok! sets one and unsets the other in one go

(note) I updated the answer after Martin’s comment below