New to sql, how to insert column values into new column?

Question

What I have:

ALTER TABLE countryb
ADD gnppercap real
;


INSERT INTO countryb (gnppercap)
    SELECT gnp/population
    FROM countryb
;

I successfully created the column "gnppercap", now I want to populate values in every row with the variable. The new variable is the product of gnp and 1/population, with variables gnp and population already in the table I'm altering, countryb.

Here's the error:

ERROR: null value in column "code" violates not-null constraint
SQL state: 23502
Detail: Failing row contains (null, null, null, null, null, null, null, null, null, null, null, >null, null, null, null, 0.000263028).

I know that the table, countryb has a ton of non-null vars in it, so that's what those nulls are, I think. I thought that since I specified the column I am inserting values into, it wouldn't matter...?

I'm lost. Help appreciated!

Answer 1

You want to update the table, not insert a new row.

update countryb set gnppercap = gnp/population

As an aside: You probably don't want to store a calculated value in a separate column. What happens if you update gnp or population? Your gnppercap column will be inaccurate.