user: Question: what is the state for reason for change is Resigned May 15, 1820?
Columns: ['State (class)', 'Vacator', 'Reason for change', 'Successor', 'Date of successors formal installation']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Alabama (2)', 'New seats', 'Alabama was admitted to the Union.', 'John W. Walker (DR)', 'Elected December 14, 1819']
SQL:

assistant: SELECT "State (class)" FROM "Table" WHERE "Reason for change" = 'Resigned May 15, 1820';