user: Question: Which Thickness has a Diameter of 21.9mm?
Columns: ['Year of Issue', 'Description', 'Weight', 'Diameter', 'Thickness', 'Edge']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [1976.0, 'Round Nickel Token, Three Maple Leaves with Beads', '4.30grams', '21.9mm', '1.71mm', 'Plain']
SQL:

assistant: SELECT "Thickness" FROM "Table" WHERE "Diameter" = '21.9mm';