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SQL Basics in PostgreSQL

Introduction
Very simple subqueries
Subqueries with multiple results
Correlated subqueries
14. The operator IN with correlated subqueries
Other subqueries

Instruction

Wow, cool! Now, remember the operator IN? It allowed us to specify a few values in the WHERE clause, so it worked a bit like the OR operator. Now, take a look:

SELECT *
FROM city
WHERE country_id IN (
  SELECT id FROM country
  WHERE country.population < 40000
);

Can you predict what the above instruction does? It shows all cities from countries where the total population of the country is less than 40,000.

Exercise

Show all information about all trips to cities which have a rating lower than 4.

Stuck? Here's a hint!

Type:

SELECT *
FROM trip
WHERE city_id IN (
  SELECT id
  FROM city
  WHERE rating < 4
);