Continuing with tutorial under SQLZOO. The second exercise is titled “SELECT from WORLD”. Using same world database from previous exercise.

New keywords to learn

XOR
Exclusive OR, it will show data and exclude another data

ROUND
It rounds the number up. It can round up by tenth, hundredth, and even thousandth. Take a look at an example below.

ROUND(7253.86, 0) -> 7254
ROUND(7253.86, 1) -> 7253.9
ROUND(7253.86,-3) -> 7000

EXERCISE 2 BEGINS!

One or the other (but not both)

I learned a new trick, using XOR. It is short for Exclusive OR, which accepts only one of either if two desires are given. Take a look at an example below:

• Australia has a big area but a small population, it should be included.
• Indonesia has a big population but a small area, it should be included.
• China has a big population and big area, it should be excluded.
• United Kingdom has a small population and a small area, it should be excluded.

Their definition of big population is >250000000 and big area as >3000000.

/* one or the other */
-- just not two BIGS together

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

RESULT:

What’s happening here is that XOR select only one that has either big population, OR big area. BIG population AND BIG area data are excluded.

Rounding

This problem wants me to:

• show the name, population in millions, and gdp in billions
• in continent ‘South America’
• use ROUND to show values in two demical places

So here goes!

SELECT name,
   ROUND(population/1000000, 2),
   ROUND(gdp/1000000000, 2)
FROM world
WHERE continent = 'South America'

Isn’t that cool?

Name and capital have the same length

There is another way to use LENGTH, which is different from other programming languages.

SELECT name, capital
FROM world
WHERE LENGTH(name) = LENGTH(capital)

What it does is searching for data of countries that has the same number of letters as their own capital.

Matching name and capital

• First letter of name and capital has to match
• name and capital cannot be the same word

In this problem, I learned the function of LEFT. It means left side and it can be used to isolate the first letter. I also learned the NOT EQUAL operator <>.

SELECT name, capital
FROM world
WHERE LEFT(name, 1) = LEFT(capital, 1) AND name <> capital

The result will list all countries and capitals that share the same first alphabetic letter and exclude these that have fully the exact same spellings.

All the vowels

• if name has more than one word, or a space in the between, reject or ignore.
• find the country that has all the vowels and no space in its name

This is the hardest problem so far. I wasn’t sure how to do it right. Now I understand how to do it properly. I thought could use commas. Nope, just has to do it in long way.

SELECT name
    FROM world
WHERE name LIKE '%a%'
    AND name LIKE  '%e%'
    AND name LIKE  '%i%'
    AND name LIKE '%o%'
    AND name LIKE '%u%'
    AND name NOT LIKE '% %'

Notice %..%, it means placeholder is anywhere. So I searched for vowels anywhere in the word. Also could use NOT LIKE to exclude the character. Only one country that has all the vowels in it is Mozambique.

Ending this exercise

Quiz should be easy! I am not going to cover them. I decided to try this Find the duplicate game. It begs the question, why and what it has to do with the SQL? I am guessing, to improve our eagle eyes against overwhelming data. It’s fun and challenging! My first score was 11, but didn’t realize that not all has to match, just keys. My second attempt, its 15! On my third try, finished the game.