...what I'd like to do is create a statement with an if/then (or even a "case" statement) that would look at the value of "brand_code" in the "item" table then enter another value like "NIKE" in the "brand_name" field in the "item_info" table. I would give the statement 10 or 15 possible entries for "brand_code" and "brand_name"?
I'm in agreement with Stacey on most of solution offered... except, if you are using a relational database system (which I am assuming) you would be replicating data.
Create your "brand" table with a code designation ("NI") and a full description ("Nike"), then in your item table reference only the brand code. If you need to find the description, join your item table to the brand code table.
This way, you are taking steps toward normalizing your data and building a better RDBMS