user: Question: when was the marriage when became dauphine is 31 August 1412?
Columns: ['Name', 'Birth', 'Marriage', 'Became Dauphine', 'Ceased to be Dauphine', 'Death', 'Husband']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Joan of Bourbon', '3 February 1338', '8 April 1350', '22 August 1350 husband became the Dauphin', '8 April 1364 became Queen', '6 February 1378', 'Charles, 1st Dauphin']
SQL:

assistant: SELECT "Marriage" FROM "Table" WHERE "Became Dauphine" = '31 August 1412';