Database design question

Hi all

When creating a table, I make an autonumber column so I can make it primary key. Then I'll use that number when making relationships. But look at the following.

I have rows that I'd like to reference as:

1.1 | some field | some field
1.2 | some field | some field
1.3 | some field | some field
1.4 | some field | some field
2.1 | some field | some field
2.2 | some field | some field
2.3 | some field | some field
2.4 | some field | some field
3.1 | some field | some field
3.2 | some field | some field
3.3 | some field | some field
etc....

What would you recommend as far as creating a primary key?
An autonumber column,
A unique decimal column (ie 2.1,2.2 etc),
or combine two indexed columns like
1 | 1 | some field | some field
1 | 2 | some field | some field
etc.

By the way, that first number in 1+n.n is a foreign key to another table.

Sorry if this sounds confusing. I just want to know if you can combine two columns to make a unique key. If you can, are there any drawbacks.

Thanks

If you plan to use auto_increment, you will need to specify the key field as INT(size) and NOT NULL. If you want to use the foreign key from another table, plus another field combined to yeild a unique key, that will work too.

I was pretty confident that using a foreign key combined with another column would be good enough as a unique key, but I'm dreading the kind of select statements I'll need to use. I will also have another table that will contain two foreign keys and another column to make a unique identifier.

Thanks for the feedback!