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_selects_population_column(self):
        """Check if only the 'population' column is selected."""
        with open("query.sql", "r") as file:
            query = file.read()
        self.cursor.execute(query)
        columns = [desc[0] for desc in self.cursor.description]
        condition = columns == ["population"]
        _dynamic_test(
            self,
            condition,
            "Query selects only the 'population' column.",
            f"Query does not select only the 'population' column. Columns found: {columns}"
        )

    def test_orders_population_ascending(self):
        """Check if the population column is ordered in ascending order (NULLs last)."""
        with open("query.sql", "r") as file:
            query = file.read()
        self.cursor.execute(query)
        results = self.cursor.fetchall()
        populations = [row[0] for row in results]
        # Remove nulls for ordering check
        non_null_populations = [p for p in populations if p is not None]
        sorted_non_null = sorted(non_null_populations)
        # Find where first null appears
        null_start = None
        for i, p in enumerate(populations):
            if p is None:
                null_start = i
                break
        # All nulls must be at the end
        nulls_at_end = True
        if null_start is not None:
            nulls_at_end = all(x is None for x in populations[null_start:])
        condition = (non_null_populations == sorted_non_null) and nulls_at_end
        _dynamic_test(
            self,
            condition,
            "Population values are ordered in ascending order with NULLs at the end.",
            "Population values are not ordered in ascending order or NULLs are not at the end."
        )

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.

Challenge: Population Sort

Solution