user: Question: Which series # had 0.645 U.S. viewers(millions)?
Columns: ['Series #', 'Episode #', 'Title', 'Directed by', 'Written by', 'Original air date', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: [27.0, 1.0, '"The Captain"', 'Clark Mathis', 'Eric Falconer', 'September21,2011', '0.891']
SQL:

assistant: SELECT "Series #" FROM "Table" WHERE "U.S. viewers (millions)" = '0.645';