If you have to access persisted data, of course, you do not have to necessarely use SQL. But in case you do, knowing something about SQL-performance helps.
I just did a this 3-minute test on SQL performance (for Oracle), with 4/5 score. Pretty fine.
The answer I failed was about multi-colomn indices. When you have a index containing multiple columns, if you use one of those columns in a where clause, the index is used. However, the index can only be used efficient if you use at first column.