set nocount on;
go
create table tFlat (
  [flat_id] int,
  [parking_id] int
)
go
create table tParking (
  [parking_id] int,
  [flag] bit
)
go
create view vRequests
with schemabinding
as
select
      f.flat_id as [flat_id],
      f.[parking_id] as [parking_id],
      p.[flag] as [request]
from [dbo].[tFlat] f join [dbo].[tParking] p on p.parking_id = f.parking_id
where p.[flag] = 1
go
create unique clustered index ix0 on vRequests ([flat_id], [parking_id], [request])
go
create view vRequests2
with schemabinding
as
select row_number() over (order by [flat_id]) as [id], [flat_id], [parking_id], [request] from [dbo].[vRequests] with(noexpand)
go

create view vRequests3 
as
with x as (
select [flat_id], [parking_id], [request] from [dbo].[vRequests] with(noexpand)
union all
select f.[flat_id], f.[parking_id], p.[flag]
from [dbo].[tFlat] f join [dbo].[tParking] p on p.parking_id = f.parking_id
where p.[flag] = 0
)
select row_number() over (order by [flat_id]) as [id], [flat_id], [parking_id], [request] from x
go

insert into tParking values (1, 1), (2, 1), (3, 0), (4, 1)
select * from vRequests2
insert into tFlat values (1, 1), (1, 2), (2, 1)
select * from vRequests2
go

insert into tParking values (5, 0)
insert into tFlat values (2, 5)
insert into tFlat values (2, 5)
select * from vRequests2
select * from vRequests3
go

insert into tFlat values (1, 5)
insert into tFlat values (1, 5)
select * from vRequests3
go
update tParking 
set flag = 1
where parking_id = 5

drop view vRequests3
drop view vRequests2
drop view vRequests
drop table tParking
drop table tFlat

set nocount on;
go
create table tFlat (
  [flat_id] int,
  [parking_id] int
)
go
create table tParking (
  [parking_id] int,
  [flag] bit
)
go

create trigger trg0 on tFlat after insert, update
as
if exists (select 1 from [dbo].[tFlat] f with(readpast)
                        join [dbo].[tParking] p on p.[parking_id] = f.[parking_id] and p.[flag] = 1
                     group by f.[flat_id], f.[parking_id]
                     having count(1) > 1) begin
                     throw 50000, 'Существуют квартиры с зарегистрированными парковками. Операция не будет выполнена', 1
end

insert into tParking values (1, 1), (2, 1), (3, 0), (4, 1)
select * from tParking
insert into tFlat values (1, 1), (1, 2), (2, 1)

insert into tFlat values (1, 2), (5, 1)
select f.flat_id, f.parking_id, p.flag from tFlat f join tParking p on p.parking_id = f.parking_id

insert into tFlat values (1, 3)
insert into tFlat values (1, 3)

insert into tFlat values (1, 4)
insert into tFlat values (1, 4)

select f.flat_id, f.parking_id, p.flag from tFlat f join tParking p on p.parking_id = f.parking_id