user: Question: What is 4ZR's callsign?
Columns: ['Callsign', 'Frequency', 'Band', 'On-air ID', 'Purpose']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['4AA', '1026', 'AM', '4MK', 'Commercial']
SQL:

assistant: SELECT "Callsign" FROM "Table" WHERE "On-air ID" = '4ZR';