대략 이런 식일 듯

if object_id('tbB') is not null
    drop table tbB
go

if object_id('tbA') is not null
    drop table tbA
go

CREATE TABLE tbA
(
    a  int  NOT NULL ,
    name  varchar(10)  NULL
)
go


ALTER TABLE tbA
    ADD CONSTRAINT  XPKtbA PRIMARY KEY   CLUSTERED (a  ASC)
go


CREATE TABLE tbB
(
    a  int  NOT NULL ,
    name  varchar(10)  NULL
)
go


ALTER TABLE tbB
    ADD CONSTRAINT  XPKtbB PRIMARY KEY   CLUSTERED (a  ASC)
go



ALTER TABLE tbB
    ADD CONSTRAINT  R_1 FOREIGN KEY (a) REFERENCES tbA(a)
go




create trigger trg_insert_tbB
on tbB
instead of insert
as
begin
    set nocount on
    insert into tbA (a, name)
        select x.*
        from inserted x
        left outer join  tbA y
        on x.a = y.a
        where
            y.a is null
   
    if @@rowcount > 0  begin
        insert into tbB
            select * from inserted
    end
end
go

insert into tbB (a, name)
    select 1, '가'
go
   
   
select * from tba
select * from tbb