Why use many columns in GROUP BY and HAVING clause in these examples - sql

Question

Given the schema here I'm trying to understand and solve the below 3 sql queries as I'm confused:
1- Present a table giving the names of the countries with ≥ 50% urbanization
rates, their urbanization rates, and their per capita GDP. Note that
urbanization rate is the percentage of population living in cities. Do not
count cities with NULL values for population.
SELECT country.name, round(sum(city.population)/country.population, 3) AS urban, round(gdp/country.population, 3) AS gdppc
FROM city
INNER JOIN country ON code = country
INNER JOIN economy ON code = economy.country
WHERE city.population IS NOT NULL
GROUP BY country.name, country.population, economy.gdp
HAVING round(sum(city.population)/country.population, 3) >= 0.5
ORDER BY urban DESC;
In the above query, Why I need to include country.population and economy.gdp in the GROUP BY? If I tried using just country.name in the GROUP BY I get an error saying I should include the others.
2- Show organizations that have as members all the European countries with over 50 million people?
SELECT name
FROM organization
INNER JOIN (SELECT organization
FROM country
INNER JOIN encompasses
ON code = encompasses.country
INNER JOIN ismember
ON code = ismember.country
WHERE population > 50000000 AND continent = 'Europe'
GROUP BY organization
HAVING count(ismember.country) = (SELECT count(*)
FROM country
INNER JOIN encompasses
ON code = country
WHERE population > 50000000 AND continent = 'Europe'))
AS innerQuery
ON abbreviation = innerQuery.organization;
Why I need the HAVING Part above?
3- Insert a new organization called “Tivoli” and a trigger that says if Germany joins “Tivoli” then so too must the UK and France. Insert Germany into the “Tivoli” organization. Confirm proper behavior.
I tried the below script but it's not working, any advice please?
do $$
begin
IF(NOT EXISTS ( SELECT 1 FROM organization WHERE organization."name" = 'Tivoli' AND organization.country = 'D' ))
BEGIN
INSERT INTO organization VALUES ('Tivoli','Tivoli organization',NULL,'F',NULL,NULL);
INSERT INTO organization VALUES ('Tivoli','Tivoli organization',NULL,'GB',NULL,NULL);
END;
end $$

Answer 1

1)
You used country.population and economy.gdp in the select, outside of aggregate functions ( COUNT(), AVG() and SUM() ), and you have a GROUP BY. Everything that you select has to be in GROUP BY or inside of aggregate functions.
2)
Because you were asked to show organizations that have ALL of 50mil + people countries. With HAVING, you check if that organization has the right amount of countries.
3)
organization."name" = 'Tivoli'
It's supposed to be :
organization.name

Answer 2

First of all, you should limit a question to one only, not 3. But here are some pointers for all 3:
In the above query, Why I need to include country.population and economy.gdp in the GROUP BY? If I tried using just country.name in the GROUP BY I get an error saying I should include the others.
This is a requirement. A group by country.name alone would work (in Postgres 9.1+) only if the other two fields are known to be functionally dependent on country.name. But probably country.name is not the primary key of the country table, so in theory it is possible to have two records in that table with the same name, but different population.
The rule is as follows:
When GROUP BY is present, it is not valid for the SELECT list expressions to refer to ungrouped columns except within aggregate functions or if the ungrouped column is functionally dependent on the grouped columns, since there would otherwise be more than one possible value to return for an ungrouped column. A functional dependency exists if the grouped columns (or a subset thereof) are the primary key of the table containing the ungrouped column.
This is implemented since version 9.1.
Why I need the HAVING Part above?
Because a condition on an aggregate (count in this case) can only be performed after grouping, and can thus not be expressed in the where clause. In this case the having clause makes sure that the organisation is not only present in some big EU Member States, but all big EU Member states.
I tried the below script but it's not working, any advice please?
Without a proper database schema, it is not possible to provide you with the correct SQ, but from the ERD diagram it seems that the organization table does not have a country field. Instead the ismember table connects organizations with countries. You would only insert one organization, but several ismember records (one per Member State involved)
It is better also to name the fields in your insert statement, so it is clear which value corresponds to which field.