user: Question: How much is the GDP for a population density of 254.7?
Columns: ['Country', 'Area (km²)', 'Population (2011 est.)', 'Population density (per km²)', 'GDP (PPP) $M USD']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Country', 'Area (km²)', 'Population (2011 est.)', 'Population density (per km²)', 'GDP (PPP) $M USD']
SQL:

assistant: SELECT "GDP (PPP) $M USD" FROM "Table" WHERE "Population density (per km²)" = '254.7';