user: Question: When William B. Campbell (U) is the successor what is the district?
Columns: ['District', 'Vacator', 'Reason for change', 'Successor', 'Date successor seated']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Tennessee 1st', 'Vacant', 'Tennessee re-admitted into the Union', 'Nathaniel G. Taylor (U)', 'July 24, 1866']
SQL:

assistant: SELECT "District" FROM "Table" WHERE "Successor" = 'William B. Campbell (U)';