Sams Teach Yourself SQL in 10 minutes - Question about GROUP BY - sql

Question

i read the book "Sams Teach Yourself SQL in 10 minutes, Third Edition" and in the lesson 10 "Grouping Data", section "Creating Groups", i can't understand the following:
"Aside from the aggregate calculations statements, every column in your SELECT statement must be present in the GROUP BY clause."
Why? I tried this and i think that it is not true.
For example, consider a table 'World' with the columns 'continent', 'country', 'population'.
SELECT continent, country
FROM World
GROUP BY continent;
According to the book, this should lead to an error, right? But it doesn't. I can group my data depending on the continent (so we have at the results 7 continents) and next to each continent, a random country name.
Like this
continent country
North America Canada
South America Brazil
Europe France
Africa Cameroon
Asia Japan
Australia New Zealand
Antarctica TuxLand

Answer 1

You are most probably using MySQL which allows ungrouped and unaggregated expressions in SELECT clause.
This is violation of standard of course.
This is intended to simplify GROUP BY with joins on a PRIMARY KEY:
SELECT a.*, SUM(b.value)
FROM a
JOIN b
ON b.a_id = a.id
GROUP BY
a.id
Normally, you would have either to add all columns from a into the GROUP BY clause or use a subquery.
MySQL allows you not to do it since all values from a are guaranteed to be the same for a given value of the PRIMARY KEY (which is grouped on).

Answer 2

This is correct and should produce no error in some forms of SQL such as MySQL. You may optionally use the GROUP BY statement on more than one column but it's not required.

Answer 3

GROUP BY will list the first result of the columns specified - so in your case, it would return the first country/continent pair.
PostgreSQL and MySQL allow this, using one field for the group by.
The tutorial probably assumes you should use GROUP BY on all fields so from what you select, you don't lose any data - it would show every country/continent in the above example, but only once.
Here's an example table:
Continent | Country | Random_Field
---------------------------------------------
North America Canada Cake
North America Canada Dog
South America Brazil Cat
Europe France Frog
Africa Cameroon House
Asia Japan Gadget
Asia India Dance
Australia New Zealand Frodo
Antarctica TuxLand Linux
In your first statement:
SELECT continent, country
FROM World
GROUP BY continent;
The output would be:
Continent | Country
--------------------------
North America Canada
South America Brazil
Europe France
Africa Cameroon
Asia Japan
Australia New Zealand
Antarctica TuxLand
Notice one of the Asia rows was lost, despite being different.
Using a GROUP BY on both:
SELECT continent, country
FROM World
GROUP BY continent, country;
Would yield:
Continent | Country
-----------------------------
North America Canada
South America Brazil
Europe France
Africa Cameroon
Asia Japan
Asia India
Australia New Zealand
Antarctica TuxLand