Create custom "auto-increment" Compound Primary Key?

Question

I have a set of parent-child tables (1 to many relationships). I'm building the tables, and have some doubts about the use of PKs and auto-increment.

Parent table has an autonumber PK (is used for storing sales ticket header). One record here means on ticket.

Child table is used for storing ticket details. One record here is one line item in the ticket (e.g. coke, mars bar, etc)

I understand that PK for child table should have 2 fields:

1. Parent tables's PK
2. A number that makes the line item unique within this ticket

If I use IDENTITY, it will not "restart" after parent's PK changes.

I'll show it with an example:

A) What SQL does

Parent table
Col1  Col2
1     1000
2     2543
3     3454
Note: Col1 is IDENTITY

Child Table
Col1  Col2  Col3
1     1     Coke
1     2     Mars Bar
2     3     Sprite
3     4     Coke
3     5     Sprite
3     6     Mars Bar
Note: Col1 is taken from Parent Table; Col2 is IDENTITY

B) What I want to achieve

Parent table is the same as above

Child Table
Col1  Col2  Col3
1     1     Coke
1     2     Mars Bar
2     1     Sprite
3     1     Coke
3     2     Sprite
3     3     Mars Bar

Note: Col1 is taken from Parent Table; Col2 resets after change in Col1; Col1 composed with Col2 are unique.

Does SQL Server implement this use of keys? Or should I need to code it?

Answer 1

Just as an example:

create table dbo.tOrders (
    OrderID int not null identity primary key,
    CustomerID int not null
);
create table dbo.tOrderPos (
    OrderID int not null foreign key references dbo.tOrders,
    OrderPosNo int null,
    ProductID int null
);
create clustered index ciOrderPos on dbo.tOrderPos
    (OrderID, OrderPosNo);
go
create trigger dbo.trInsertOrderPos on dbo.tOrderPos for insert
as begin
    update  opo
    set     OrderPosNo = isnull(opo2.MaxOrderPosNo,0) + opo.RowNo
    from    (select OrderID, OrderPosNo,
                    RowNo = row_number() over (partition by OrderID order by (select 1))
            from    dbo.tOrderPos opo
            where   OrderPosNo is null) opo
    cross apply
            (select MaxOrderPosNo = max(opo2.OrderPosNo)
            from    dbo.tOrderPos opo2
            where   opo2.OrderID = opo.OrderID) opo2
    where   exists (select * from inserted i where i.OrderID = opo.OrderID);
end;
go
declare @OrderID1 int;
declare @OrderID2 int;
insert into dbo.tOrders (CustomerID) values (11);
set @OrderID1 = scope_identity();
insert into dbo.tOrderPos (OrderID, ProductID)
values (@OrderID1, 1), (@OrderID1, 2), (@OrderID1, 3);
insert into dbo.tOrders (CustomerID) values (12);
set @OrderID2 = scope_identity();
insert into dbo.tOrderPos (OrderID, ProductID)
values (@OrderID2, 4), (@OrderID2, 5);
insert into dbo.tOrderPos (OrderID, ProductID)
values (@OrderID1, 6);
select * from dbo.tOrderPos;
go
drop trigger dbo.trInsertOrderPos;
drop table dbo.tOrderPos;
drop table dbo.tOrders;
go

The difficulty has been to allow multiple inserts and delayed inserts. HTH

Another option is using an instead-of-trigger:

create trigger dbo.trInsertOrderPos on dbo.tOrderPos instead of insert
as begin
    insert into dbo.tOrderPos
            (OrderID, OrderPosNo, ProductID)
    select  OrderID,
            OrderPosNo =
            isnull( (select max(opo.OrderPosNo)
                    from    dbo.tOrderPos opo
                    where   opo.OrderID = i.OrderID), 0) +
            row_number() over (partition by OrderID order by (select 1)),
            ProductID
    from    inserted i;
end;

Unfortunately it doesn't seem to be possible to set the OrderPosNo "not null" because multiple inserts would lead to a duplicate key. Therefor I couldn't use a primary key and used a clustered index instead.

Answer 2

You don't have a one-to-many relationship. You have a many-to-many relationship. A parent can have many items. A coke can belong to more than one parent.

You want three tables. The in-between table is sometimes called a junction table.

http://en.wikipedia.org/wiki/Junction_table

Note: In the wiki article they only show two columns in the junction table, I believe a best practice is for that table to also have a unique auto-incrementing field.

Note: The two joining fields are usually made a unique index.