Summary  
This chapter explains how to use the OR boolean operator in a WHERE clause to filter records based on multiple conditions in a SQL query.

General domain of usage  
relational database querying

The `OR` operator is used to filter records based on more than one condition, like if you want to return all countries from Asia but also those from Europe.

Here is the syntax of using the OR operator:

```sql
SELECT columns
FROM table
WHERE column = 'first_option' OR column = 'second_option';
```

Let's look at the example:

SELECT name, population, continent, region
FROM country
WHERE continent = 'Asia' OR continent = 'Europe';

import unittest
import psycopg2
import pandas as pd
import re

class TestPostgresQuery(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        cls.connection = psycopg2.connect(
            host="localhost",
            user="agent",
            password="qwerty",
            dbname="testdb",
            sslmode="disable"
        )
        cls.cursor = cls.connection.cursor()
    
    def test_select_name_from_country_or_operator(self):
        """Check query retrieves correct country names from Asia or North America using OR operator."""
        with open("query.sql", "r") as file:
            query = file.read()
        # Reference solution
        solution_query = """
        SELECT name
        FROM country
        WHERE continent = 'Asia' OR continent = 'North America';
        """
        self.cursor.execute(solution_query)
        expected_rows = self.cursor.fetchall()
        expected_names = set([row[0].strip() for row in expected_rows])
        self.cursor.execute(query)
        user_rows = self.cursor.fetchall()
        user_names = set([row[0].strip() for row in user_rows])
        condition = user_names == expected_names
        missing = expected_names - user_names
        extra = user_names - expected_names
        if not condition:
            failure_msg = f"Expected country names: {sorted(expected_names)}, but got: {sorted(user_names)}.\\nMissing: {sorted(missing)}\\nExtra: {sorted(extra)}"
        else:
            failure_msg = ""
        _dynamic_test(
            self,
            condition,
            "Query returns the correct country names from Asia or North America.",
            failure_msg
        )

    def test_query_uses_where_and_or(self):
        """Check that WHERE and OR are used in the query."""
        with open("query.sql", "r") as file:
            query = file.read().lower()
        has_where = "where" in query
        has_or = "or" in query
        condition = has_where and has_or
        _dynamic_test(
            self,
            condition,
            "Query uses WHERE clause and OR operator.",
            "Query must use both WHERE clause and OR operator."
        )

def _dynamic_test(test_case, condition, success_message, failure_message):
    if condition:
        test_case._testMethodName = success_message
        test_case.assertTrue(True, success_message)
    else:
        test_case._testMethodName = failure_message
        test_case.fail(failure_message)

def normalize_text(text):
    text = text.lower()
    text = re.sub(r"\s{2,}", " ", text)
    text = re.sub(r"\s*([,:?])\s*", r"\1 ", text)
    return text.strip()

if __name__ == "__main__":
    unittest.main()


test_solution.py

Learn the core fundamentals of SQL and relational databases. Understand how databases store and organize data, write basic SQL queries, work with tables and perform essential operations. Build a strong foundation for working with databases in backend development.

Using the OR operator

Soluzione