Table: singapore_airlines
Columns: airlines
iata
icao
callsign
commenced_operations

Question: Who is the callsign that has ICAO of SCO?
SQL: SELECT "callsign" FROM singapore_airlines WHERE "icao" = 'sco'

Question: Who is the commenced operations that has icao of slk?
SQL: SELECT "commenced_operations" FROM singapore_airlines WHERE "icao" = 'slk'

Question: Who is the call sign that has a commenced operation before 1976?
SQL: SELECT "callsign" FROM singapore_airlines WHERE "commenced_operations" < 1976