Combine two integers to create a unique number

Question

Good Morning,

I was looking for a way to combine two integers to create a unique number, I have two tables that I need to combine into a third table with unique numb

Answer 1

Similar to Magnus Hoff, but I would recommend using a binary friendly approach instead of a base 10 approach.

combinedid = (classid << 8) + schoolid;

And then, later:

classid = combinedid >> 8;
schoolid = combinedid & 0xFF;

I think this is a little more straight forward from a programming standpoint (making it clear that your school ID is 1 byte (0-255), the class ID is 3 bytes).

You could also easily do this with a bigint (Long / Int64), making two int32's a single int64 safely:

combinedid = ((long)classid << 32) + schoolid;

Answer 2

Use ((a + b)(a + b + 1) * 0.5) + b Reference http://en.wikipedia.org/wiki/Pairing_function

Answer 3

combinedid = classid*1000 + schoolid

And then, later:

classid = combinedid / 1000 // Integer division
schoolid = combinedid % 1000

Answer 4

If SchoolID is always a 3 digit number multiply ClassId by 1000 then add SchoolID.

Your number might "overflow" though., and given the ones you have if they are 32-bit they will.

Answer 5

You indicate that you are working with a table. This leads me to believe that you are working within a database.

So I have to ask, why not store them in separate columns and make them simple foreign keys? Why create the ambiguity of trying to concatenate or transform the numbers with a math equation?

If you use another table, you can use an auto increment field and the combination of the three fields (in the order you designate) will give you a unique ID.

|-------------------
| My_combine_table
|-------------------
| id   | auto_inc 
|-------------------
| SchoolID | ...
| SchoolID2 | ...

Answer 6

This SO thread has detailed various other mathematical approaches, some are just better. But for your case I suspect if those are hardly good ideas. I would say you should save the two ids in a 3rd table which has its own unique id (primary key) column.