Question

This question is geared towards MySQL, since that is what I’m using — but I think that it’s probably the same or similar for almost every major database implementation.

How do keys work in a database? By that I mean, when you set a field to ‘primary key’, ‘unique key’ or an ‘index’ — what do each of these do, and when should I use each one?

Right now I have a table containing a few fields, one of them being a GUID (minus the { and } around it). I set the GUID field to the primary key and I see that it created a binary tree. So it improves search performance — but what differentiates that from other types of keys?

I realize this may not really be programming related (although it is development related) — I wasn’t sure where exactly to ask this but SO is what I use the most so I’ll ask here. Migrate as necessary

Answer 1

There are probably hundreds of references for this elsewhere on the web, so a bit of Googling will help you get deep into understanding DB design. That said, the basic gist is:

• primary key: a field or combination of fields which must be unique for each row, and which is/are indexed to provide rapid lookup of a row given a key value; cannot contain NULL, and a table can only have one primary key. Generally indexed in a clustered index, which means that the data in the table is reordered to match the order of the index, a process that greatly improves serial data retrieval. (This is the main reason a table can only have one primary key — the order of the data can’t match the order of more than one index!)
• unique key: same as a primary key, but on some DB platforms, can contain NULL values so long as they don’t violate the uniqueness constraint. (In other words, if the unique key contains a single column, there can only be one row in the table with NULL in that column; if the key contains more than one column, then the table can only contain rows with NULLs in the columns such that there’s no non-unique duplication of NULL values across the columns in the key.) On other platforms (including MySQL), unique constraints can contain multiple NULLs; the uniqueness constraint only applies to non-NULL values of the referenced columns. There can be more than one of these per table. Indexed in a non-clustered index.
• index: a field or combination of fields which are pre-indexed for more rapid retrieval given a value for the field(s) in the index. A table can have more than one index.