user: Question: When did the institution located in Lewiston, Maine join the conference? 
Columns: ['Institution', 'Location', 'Nickname', 'Founded', 'Founding Religious Affiliation', 'Enrollment', 'Joined']
Types: ['text', 'text', 'text', 'real', 'text', 'real', 'real']
Sample row: ['Amherst College', 'Amherst, Massachusetts', 'Lord Jeffs', 1821.0, 'Congregationalist', 1817.0, 1971.0]
SQL:

assistant: SELECT "Joined" FROM "Table" WHERE "Location" = 'Lewiston, Maine';