Database “key/ID” design ideas, Surrogate Key, Primary Key, etc

Question

So I\'ve seen several mentions of a surrogate key lately, and I\'m not really sure what it is and how it differs from a primary key.

I always assumed that ID was my prim

Answer 1

There are actually three kinds of keys to talk about. The primary key is what is used to uniquely identify every row in a table. The surrogate key is an artificial key that is created with that property. A natural key is a primary key which is derived from the actual real life data.

In some cases the natural key may be unwieldy so a surrogate key may be created to be used as a foreign key, etc. For example, in a log or diary the PK might be the date, time, and the full text of the entry (if it is possible to add two entries at the exact same time). Obviously it would be a bad idea to use all of that every time that you wanted to identify a row, so you might make a "log id". It might be a sequential number (the most common) or it might be the date plus a sequential number (like 20091222001) or it might be something else. Some natural keys may work well as a primary key though, such as vehicle VIN numbers, student ID numbers (if they are not reused), or in the case of joining tables the PKs of the two tables being joined.

This is just an overview of table key selection. There's a lot to consider, although in most shops you'll find that they go with, "add an identity column to every table and that's our primary key". You then get all of the problems that go with that.

In your case I think that a LogEntryID for your log items seems reasonable. Is the ID an FK to the Users table? If not then I might question having both the ID and the LogEntryID in the same table as they are redundant. If it is, then I would change the name to user_id or something similar.