Primary vs Unique ID : Copy field to second table

LiquidCD

I have a question regarding the function of the primary key and the unique action for databases.

I have a membership database in which a userid is auto_incremented whenever a new member is put in to the database. The userid field is listed as a primary and unique key field...but what exactly does it mean?

Also, is it possible to take the userid of a member, and place the same userid into a second table that matches the same field?

Example: The membership table has the userid, first name, last name and email fields. The surname table has mem_id, surname and email fields.

I would like to compare the email field from the membership table and the surname table. If the email fields match, I want to take the userid field and input the same value into the mem_id field of the second table. Is this possible?

Thanks in advance!

ahundiak

You might want to take a look at this article.
http://www.phpbuilder.com/columns/barry20000731.php3

In spite of it's rather imposing sounding title, the article is acutally a good introduction to using indexes.

A primay key is a column that uniquely identifies a given row in a table. It's usually a number but does not have to be. Primary keys are always unique. Given the key you can be assurred of getting one and only one record.

You can indeed add a userid to the surname table. In this case, userid now becomes a foriegn key which basically means that it refers to a record in a different table. You will probably still want to have another primary key for the surname table.

It's not real clear to me what the relation between surname and membership is. But I think if you work through the examples in the article then you see how to join them.