What is best for Primary key

[tagteam]

I have a database that has several tables
Railcars
Customers
Assignments
Contacts

Each railcar is unique with a Initial and Number. Is it best to use these two together to make a primary key or is it better to just assign each railcar an uniqu ID number?
Thanks

[June7]

I don't like compound (composite) keys but that is my personal preference. Two fields isn't so bad but that's about as far as I would permit if I absolutely had to use. I have one table that would use 5 fields as unique identifier. In this case I went with Autonumber field as pk.

In my biggest db (the main reason for my job) is a program generated identifier as primary key. This is a 'number' assigned to a material sample logged in for testing. The structure is YYYYA-####. The sequential number part starts over from 1 each year. In 30 years this ID structure has not failed. Never exceeded 4500 in any year. Not my design but it works for our requirements.

[orange]

Further to June7's comment, a Primary key is primarily for use by the Database software. If the primary key you are considering can change (ever), then use another construct for the Primary Key. It is very possible to have a system assigned autonumber (guaranteed unique by the database software) and still have the fields that mean something to you --initial + railcarNumber -- for your use (just not the primary key). If there is a number welded onto the railcar, that may be the primary key. Often when the developer concocts a "string" with some built-in intelligence --Number - letter-number 0-9 etc to be a PK, it fails because some condition evolves and negates that key/string (no longer unique or some part of the string no longer applies).

Lots to think about when selecting the PK.
Good luck.

Here's a tutorial that you may find useful
http://www.rogersaccesslibrary.com/T...lationship.zip

If you are mathematical, there is a free video with more of the "rationale" and "meaning" of Primary Keys and relations at
http://shop.oreilly.com/product/0636920002710.do (It the free first part of others -very good but based in mathematics)
NOTE: Autonumbers are not necessarily incremental nor positive-- they are unique)

Every table has a primary key that uniquely identifies each row in that table.

[tagteam]

Thank orange, that was a good read.
The car number initial combo will never change - if it does it would be entered as a new car. However, I think I will go with a single autonumber field to make sure it is as clean and simple as possible.
thanks for the hellp