user: Question: When was incumbent William B. Oliver first elected?
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Result', 'Candidates']
Types: ['text', 'text', 'text', 'real', 'text', 'text']
Sample row: ['Alabama 1', 'John McDuffie', 'Democratic', 1918.0, 'Re-elected', 'John McDuffie (D) 84.0% Aubrey Boyles (R) 16.0%']
SQL:

assistant: SELECT "First elected" FROM "Table" WHERE "Incumbent" = 'William B. Oliver';