Table: tennis_matches
Columns: tournament
date
city
country
surface
singles_champions
doubles_champions

Question: How many doubles champions were there for the Serbia f6 Futures tournament?
SQL: SELECT COUNT("doubles_champions") FROM tennis_matches WHERE "tournament" = 'Serbia F6 Futures'

Question: How many singles tournaments did Ludovic Walter win?
SQL: SELECT COUNT("tournament") FROM tennis_matches WHERE "singles_champions" = 'Ludovic Walter'