Summary  
This chapter explains how to use SQL’s WHERE clause with comparison operators (such as =, <>, <, <=, >, >=, BETWEEN, and IS NULL) to filter query results.  

General domain of usage  
Relational database querying

SQL supports a lot of conditional statements.



## Less Than

Let's look at the examples: <br>
This example lists all countries that have a population of **less than** 2424200:

SELECT name, population
FROM country
WHERE population < 2424200;

## Less Than or Equal to

The following example retrieves all countries with a population **less than or equal to** 2424200:

SELECT name, population
FROM country
WHERE population <= 2424200;

## Not Equal

The following example retrieves all non-Asian countries:


SELECT name, continent
FROM country
WHERE continent<>'Asia';

import unittest
import psycopg2
import pandas as pd
import re

class TestTask(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 setUp(self):
        with open("query.sql", "r") as file:
            self.query = file.read()

    def test_columns_and_order(self):
        """Check that the selected columns are correct and in the correct order."""
        self.cursor.execute(self.query)
        columns = [desc[0] for desc in self.cursor.description]
        expected_columns = ["name", "population", "region", "capital"]
        condition = columns == expected_columns
        _dynamic_test(
            self,
            condition,
            "Columns are correct and in the correct order.",
            f"Columns are incorrect or not in the correct order. Expected: {expected_columns}, Got: {columns}"
        )

    def test_only_southern_europe(self):
        """Check that only rows with region 'Southern Europe' are returned."""
        self.cursor.execute(self.query)
        rows = self.cursor.fetchall()
        idx_region = [desc[0] for desc in self.cursor.description].index("region")
        incorrect_rows = [row for row in rows if normalize_text(str(row[idx_region])) != normalize_text("Southern Europe")]
        condition = len(incorrect_rows) == 0
        _dynamic_test(
            self,
            condition,
            "All rows have region 'Southern Europe'.",
            f"Rows found with region not 'Southern Europe': {incorrect_rows}"
        )

    def test_expected_rows(self):
        """Check that all expected rows for 'Southern Europe' are present."""
        # Prepare solution for expected rows
        solution_query = """
        SELECT name, population, region, capital
        FROM country
        WHERE region='Southern Europe';
        """
        self.cursor.execute(solution_query)
        expected_rows = self.cursor.fetchall()

        self.cursor.execute(self.query)
        user_rows = self.cursor.fetchall()
        # Compare as sets for unordered row comparison
        condition = set(user_rows) == set(expected_rows)
        missing = set(expected_rows) - set(user_rows)
        extra = set(user_rows) - set(expected_rows)
        if not condition:
            failure_msg = f"Missing rows: {missing}\\nExtra rows: {extra}"
        else:
            failure_msg = ""
        _dynamic_test(
            self,
            condition,
            "All expected rows for 'Southern Europe' are present and no extra rows.",
            failure_msg
        )

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.

The WHERE Clause Operators

Less Than

Less Than or Equal to

Not Equal

Solution