user: Question: What is the average year with Nathan Robertson Gail Emms in mixed doubles?
Columns: ['Year', "Men's singles", "Women's singles", "Men's doubles", "Women's doubles", 'Mixed doubles']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [1984.0, 'Icuk Sugiarto', 'Helen Troke', 'Christian Hadinata Hadibowo', 'Gillian Gilks Karen Beckman', 'no competition']
SQL:

assistant: SELECT AVG("Year") FROM "Table" WHERE "Mixed doubles" = 'Nathan Robertson Gail Emms';