autoincrement for composite key

[Aditi Andurkar]

Hello all,

I have created a table data having composite key (id1,id2,id3). I want to autoincrement id3 but tht should depend on id1, id2. For example,
id1 id2 id3
1 1 1
1 1 2
1 1 3
1 2 1
1 2 2
1 1 4
As you can see, as soon as any of id1 or id2 changes id3 starts from 1.
Can anyone tell me how to write create table query for this scenario?

Waiting for a reply.
Regards,
Adi