为什么不是允许行插入的触发器

Question

试图找出当客人数量超过座位数时触发器允许将行插入座位表的原因。这是一个单行级触发器，我正在试验并在SO的帮助下进行自学。

create table dining_table
(
table_id                int             not null,
area_id                 int             not null,
table_seats             int             not null

constraint PK_dining_table primary key(table_id),
constraint FK_dining_table_table_area foreign key(area_id)
    references table_area(area_id)
);

insert into dining_table(table_id, area_id, table_seats)
values(1, 1, 2);
insert into dining_table(table_id, area_id, table_seats)
values(2, 2, 4);

create table customers
(
customer_id             int             not nulL,
cust_lastname           varchar(50)     not null,
cust_total_guests       int             not null,

constraint PK_customers primary key(customer_id)
);

insert into customers(customer_id, cust_lastname, cust_total_guests)
values(1, 'Wallman', 4);
insert into customers(customer_id, cust_lastname, cust_total_guests)
values(2, 'Griffin', 6);


create table seating
(
customer_id             int             not null,
table_id                int             not null,
seat_cost               decimal(10,2)   not null,
seat_date               datetime        not null,
tip_amount              decimal(10,2)   not null,

constraint PK_seating primary key(customer_id, table_id)    
);

insert into seating(customer_id, table_id, seat_cost, seat_date,   
tip_amount)
values(1, 1, 15.00, '20170506', 5.00);
insert into seating(customer_id, table_id, seat_cost, seat_date, 
tip_amount)
values(2, 2, 35.50, '20170608', 11.00);

触发：

if OBJECT_ID('seats_available','U') is not null
drop trigger seats_available;
go

create trigger seats_available
on dbo.seating
instead of insert 
as
begin
    insert into seating
    select *
    from inserted
    where exists (select dt.table_seats 
                  from dining_table dt
                  join seating s on dt.table_id = s.table_id
                  join customers c on s.customer_id = c.customer_id
                  where dt.table_seats >= c.cust_total_guests)
end
go
--test trigger

insert into seating(customer_id, table_id, seat_cost, seat_date,  
tip_amount)
values(2, 1, 25.00, GETDATE(), 3.00);



values(2, 2, 35.50, '20170608', 11.00);