Computing new columns

The select can compute new columns base on what's already in the table. Here for example we're computing the population density. You can add an "as" command to rename the resulting column, here we're calling it population_density.

Give the name and the per capita GDP of all countries in South America

To calculate the per capita GDP, divide the "gdp" column by the "population" column. Also, you'll notice that we formatted the starting query a bit differently here, once your queries get a bit complex, it can be a good idea to make them span multiple lines.