Tag Archives: learning

I was feeling good about the speed that I was progressing through the early SQLzoo SQL tutorials. Then I hit question eight in the “SELECT FROM world” tutorial, and couldn’t work out what response the question was seeking. It seems from the various help threads on StackOverflow and Reddit that I am not the only one who has had this issue.

To my mind, the phrasing of the question implies that there is an XOR feature in SQL, as there is in many other languages. So first I tried using the same format as the OR-based answer to the previous question, but changing the operator:

SELECT name, population, area
FROM world
WHERE area > 3000000 XOR population > 250000000

This didn’t work. So then I think “maybe there is no XOR operator in SQL and I have to do this manually”. So I try:

SELECT name, area, population
FROM   world
WHERE  (area > 3000000 AND population <= 250000000)
OR     (area <= 3000000 AND population > 250000000)

This gives the following error:

Wrong answer. Some of the data is incorrect.

But the data are not incorrect. Only after some fretting did I realise that SQLzoo wants you to SELECT the population before you call the area. Hence the correct solution is:

SELECT name, population, area
FROM   world
WHERE  (area > 3000000 AND population <= 250000000)
OR     (area <= 3000000 AND population > 250000000)

This got me the “Correct answer” result, and the weird happy-face emoji. Onwards!