Michel,

>Now, what if I need to extract the minimum value between the primary key and the secondary key. Because, on some results, it may be that the result includes a record which is having the secondary key higher than the primary key.

You lost me here. What result do you want if we change your first example to this:

000001 000002
000002 000003
000003 000003
000004 000002
000005 000002
000006 000003