user: Question: What is the average year joined of the 277 county?
Columns: ['School', 'Location', 'Mascot', 'County', 'Enrollment', 'IHSAA Class', 'Year Joined', 'Previous Conference']
Types: ['text', 'text', 'text', 'real', 'text', 'text', 'real', 'text']
Sample row: ['Borden *', 'Borden', 'Braves', 228.0, 'A', '10 Clark', 1974.0, 'Lost River']
SQL:

assistant: SELECT AVG("Year Joined") FROM "Table" WHERE "County" = 277.0;