user: Question: who is the the incumbent with candidates being Percy E. Quin (D) Unopposed
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Result', 'Candidates']
Types: ['text', 'text', 'text', 'real', 'text', 'text']
Sample row: ['Mississippi 1', 'John E. Rankin', 'Democratic', 1920.0, 'Re-elected', 'John E. Rankin (D) Unopposed']
SQL:

assistant: SELECT "Incumbent" FROM "Table" WHERE "Candidates" = 'Percy E. Quin (D) Unopposed';