user: Question: What date did the institution that left in 2013 and that is nicknamed Corsairs, join? 
Columns: ['Institution', 'Location', 'Founded', 'Type', 'Enrollment', 'Nickname', 'Joined', 'Left', 'Football Conference', 'Primary Conference']
Types: ['text', 'text', 'real', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Boston State College', 'Boston, Massachusetts', 1872.0, 'Public', 'N/A', 'Beacons', '1972', '1982', 'Merged into the University of Massachusetts Boston', 'Merged into the University of Massachusetts Boston']
SQL:

assistant: SELECT "Joined" FROM "Table" WHERE "Left" = '2013' AND "Nickname" = 'Corsairs';