user: Question: Which Marriage has a Became Duchess of 12 December 1666?
Columns: ['Name', 'Birth', 'Marriage', 'Became Duchess', 'Ceased to be Duchess', 'Death', 'Spouse']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Eleanor Gonzaga', '18 November 1630', '30 April 1651', "9 July 1654 husband's accession", "2 April 1657 husband's death", '6 December 1686', 'Ferdinand I']
SQL:

assistant: SELECT "Marriage" FROM "Table" WHERE "Became Duchess" = '12 December 1666';