user: Question: Who played Mixed Doubles in 1991?
Columns: ['Year', 'Mens singles', 'Womens singles', 'Mens doubles', 'Womens doubles', 'Mixed doubles']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [2013.0, 'Lin Dan', 'Ratchanok Inthanon', 'Muhammad Ahsan Hendra Setiawan', 'Wang Xiaoli Yu Yang', 'Tontowi Ahmad Lilyana Natsir']
SQL:

assistant: SELECT "Mixed doubles" FROM "Table" WHERE "Year" = 1991.0;