Table: zemlja_match_stats
Columns: edition
round
date
against
surface
opponent
result
outcome

Question: If the date is 7-21-2006, who was the opponent?
SQL: SELECT "opponent" FROM zemlja_match_stats WHERE "date" = '7-21-2006'

Question: If the opponent is Kristian Pless, who was it against?
SQL: SELECT "against" FROM zemlja_match_stats WHERE "opponent" = 'Kristian Pless'