Postgres 14

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select version();

version

PostgreSQL 14.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-10), 64-bit

SELECT 1

create table application( bank     integer not null
                         , mortage integer not null 
                         , status  text    not null
                         ) ;

CREATE TABLE

create or replace function just_1_non_draft()
  returns  trigger 
  language plpgsql
as $$
begin  
    if exists (select null 
                 from application 
                where (bank, mortage) = (new.bank,new.mortage) 
                  and status != 'draft'        
              ) 
        then 
           raise exception 'Bank %, Mortage % already has non-draft status',new.bank, new.mortage;
    end if; 
    return new;   
end;
$$;

create trigger just_1_non_draft_biur
    before insert or update of status 
        on application
       for each row 
      when (new.status != 'draft') 
   execute function just_1_non_draft(); 

CREATE FUNCTION

CREATE TRIGGER

insert into application( bank, mortage, status)
   values (1, 2, 'draft');

 insert into application( bank, mortage, status)
   values (1, 2, 'draft');
   
insert into application( bank, mortage, status)
  values (1, 2, 'active');

INSERT 0 1

INSERT 0 1

INSERT 0 1

insert into application( bank, mortage, status)
  values (1, 2, 'in progress');

ERROR:  Bank 1, Mortage 2 already has non-draft status
CONTEXT:  PL/pgSQL function just_1_non_draft() line 9 at RAISE

insert into application( bank, mortage, status)
  values (3, 2, 'active');

INSERT 0 1

select * from application;

bank
mortage
status

1
2
draft

1
2
draft

1
2
active

3
2
active

SELECT 4