user: Question: For how many years has the mercury prize been awarded?
Columns: ['Year', 'Award', 'Category', 'Nominated Work', 'Result']
Types: ['real', 'text', 'text', 'text', 'text']
Sample row: [2008.0, 'MOJO Awards', 'Breakthrough Artist', 'Themselves', 'Won']
SQL:

assistant: SELECT COUNT("Year") FROM "Table" WHERE "Award" = 'mercury prize';