user: Question: What is Population density (per km²), when Name is Antigua and Barbuda, and when Capital is St. John's?
Columns: ['Name', 'Subdivisions', 'Area (km²)', 'Population (1 July 2005 est.)', 'Population density (per km²)', 'Capital']
Types: ['text', 'text', 'real', 'real', 'real', 'text']
Sample row: ['Antigua and Barbuda', 'Parishes', 440.0, 85632.0, 195.0, "St. John's"]
SQL:

assistant: SELECT "Population density (per km²)" FROM "Table" WHERE "Name" = 'Antigua and Barbuda' AND "Capital" = 'St. John''s';