user: ### NOW ANSWER:
Question: Who won the womens singles when Marc Zwiebler won the men's singles?
Columns: ['Year', 'Mens singles', 'Womens singles', 'Mens doubles', 'Womens doubles', 'Mixed doubles']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [2012.0, 'Chou Tien-chen', 'Sashina Vignes Waran', 'Ruud Bosch Koen Ridder', 'Samantha Barning Eefje Muskens', 'Jorrit de Ruiter Samantha Barning']
SQL:

assistant: SELECT "Womens singles" FROM "Table" WHERE "Mens singles" = 'Marc Zwiebler';