I am building a poker application as a hobby project. I am building it's functionality in a two-step process.

Step 1: Write an application that can extract poker data from existing XML documents or postgreSQL databases.

Step 2: Expand the application's functionality so I can create my own tailored SQL databases with it.

The information I intend to extract from databases and maybe write to my own databases will look like:

playernameB... value1... value2... value3... value4

I will always query information from the database based on the playernames. The function calls might look like:
getstat(playernameA, statvalue1)
getstat(playernameA, statvalue1, statvalue2)
getstat(playernameA, allStats)

I would say that the data I want to extract and store is not very complexly relational.

When storing this information, how much will I gain in speed (or other factors I haven't considered) if I use SQL relational databases compared to XML "databases". The reason I'm asking is that I pretty much know how to create queries like this using XML, but I have no idea how to do it in SQL. So I'm wondering whether I ought to learn SQL or not.