user: Question: tell me the total number of decile with a roll showing 251.
Columns: ['Name', 'Years', 'Gender', 'Area', 'Authority', 'Decile', 'Roll']
Types: ['text', 'text', 'text', 'text', 'text', 'real', 'real']
Sample row: ['Featherston School', '1–8', 'Coed', 'Featherston', 'State', 3.0, 64.0]
SQL:

assistant: SELECT COUNT("Decile") FROM "Table" WHERE "Roll" = 251.0;