CREATE TABLE table_37212 ("Name" CLOB, "Birth" CLOB, "Marriage" CLOB, "Became Duchess" CLOB, "Ceased to be Duchess" CLOB, "Death" CLOB, "Spouse" CLOB)

Which Marriage has a Became Duchess of 12 december 1666?
SELECT "Marriage" FROM table_37212 WHERE "Became Duchess" = '12 december 1666'
SELECT "Marriage" FROM "table_37212" WHERE "Became Duchess" = '12 december 1666'