Table: airlines_changi
Columns: airline
iata
icao
callsign
commenced_operations

Question: Which Callsign has an IATA of tr?
SQL: SELECT "callsign" FROM airlines_changi WHERE "iata" = 'tr'

Question: Which Commenced operations have an Airline of valuair?
SQL: SELECT "commenced_operations" FROM airlines_changi WHERE "airline" = 'valuair'

Question: Which Commenced operations have an Airline of valuair?
SQL: SELECT MAX("commenced_operations") FROM airlines_changi WHERE "airline" = 'valuair'

Question: Which IATA has a ICAO of slk?
SQL: SELECT "iata" FROM airlines_changi WHERE "icao" = 'slk'