A subquery with the ALL operator - sql

Question

I'm trying to write a SQL (Postgres engine) query that answers the following question:
Which countries have a GDP greater than every country in Asia? [Give the name only.] (Some countries may have NULL gdp values)
Below is an abbreviated SQL table containing sample data.
+-------------+-----------+---------+------------+--------------+
| name | continent | area | population | gdp |
+-------------+-----------+---------+------------+--------------+
| Afghanistan | Asia | 652230 | 25500100 | 20343000000 |
| Albania | Europe | 28748 | 2831741 | 12960000000 |
| Algeria | Africa | 2381741 | 37100000 | 188681000000 |
| Andorra | Europe | 468 | 78115 | 3712000000 |
| Angola | Africa | 1246700 | 20609294 | 100990000000 |
+-------------+-----------+---------+------------+--------------+
I wrote something like the following, which returns nothing (although I know this isn't the answer due to the online interactive guide I'm using):
SELECT name
FROM world
WHERE gdp > ALL (SELECT gdp from world WHERE continent = 'Asia')
AND continent<>'Asia'
What query should I use? I'm pretty new to SQL.

Answer 1

Strictly speaking, if there is a single Asian country in your table with an unknown (NULL) GDP, then the correct answer must be:
"We do not know."
Accordingly, this query returns no row:
SELECT name
FROM world
WHERE gdp > ALL (SELECT gdp from world WHERE continent = 'Asia')
AND continent <> 'Asia';
A NULL value in the set of Asian GDP's makes it impossible for the first WHERE expression to be TRUE, so the query can never return rows - which is the correct answer. There is no country that we know to fit the requirement. The query you had is correct.
The variant you squeezed into a comment comparing to max(gdp) answers a slightly different question:
Which countries have a GDP greater than the greatest known GDP in Asia?

Answer 2

It sounds like you want the sum of all Asian countries. If so, try something like this:
select w1.name
from world w1
where w1.gdp > (
select sum(coalesce(w2.gdp,0))
from world w2
where w2.continent='Asia' )
If not, and you want just the single Asian country with the highest GDP, replace sum with max. You don't need the additional continent <> 'Asia' because no Asian country can have a higher GDP than the returned value.

Answer 3

Based on #Tony 's answer, it should be like this:
select w1.name
from world w1
where w1.gdp > (
select max(coalesce(w2.gdp,0))
from world w2
where w2.continent='Asia' )
and w2.gdp is not null;
Why use max instead of sum? Because the question asked:
Which countries have a GDP greater than every country in Asia?
So finding the largest gdp in Asia will work and you don't have to compare with all of the countries in Asia.
I am sorry I can't comment on #Tony 's answer since my reputation is under 50.