import unittest
import psycopg2
import pandas as pd

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_capital_column(self):
        """Check if query selects only the 'capital' column from country."""
        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 == ["capital"]
        _dynamic_test(
            self,
            condition,
            "Query selects only the 'capital' column.",
            f"Query should return only the 'capital' column, but got: {columns}"
        )

    def test_uses_country_table(self):
        """Check if data comes from the country table."""
        # We'll check if all results are among the capitals from the country table
        with open("query.sql", "r") as file:
            query = file.read()
        # Get expected capitals from country
        self.cursor.execute("SELECT capital FROM country")
        expected_capitals = set(row[0] for row in self.cursor.fetchall())
        self.cursor.execute(query)
        result_capitals = set(row[0] for row in self.cursor.fetchall())
        condition = result_capitals.issubset(expected_capitals)
        _dynamic_test(
            self,
            condition,
            "Query retrieves capitals from the correct table.",
            "Query result includes values not in the 'country' table's 'capital' column."
        )

    def test_orders_by_capital_desc(self):
        """Check if results are sorted by capital in descending order."""
        with open("query.sql", "r") as file:
            query = file.read()
        self.cursor.execute(query)
        result = [row[0] for row in self.cursor.fetchall()]
        sorted_desc = sorted(result, reverse=True)
        condition = result == sorted_desc
        _dynamic_test(
            self,
            condition,
            "Results are sorted by 'capital' in descending order.",
            f"Results are not sorted in descending order: {result}"
        )

    def test_all_capitals_included(self):
        """Check if all capitals from country table are included in result (including NULLs if present)."""
        with open("query.sql", "r") as file:
            query = file.read()
        self.cursor.execute("SELECT capital FROM country")
        expected = set(row[0] for row in self.cursor.fetchall())
        self.cursor.execute(query)
        result = set(row[0] for row in self.cursor.fetchall())
        condition = expected == result
        _dynamic_test(
            self,
            condition,
            "All capitals from the country table are included in the result.",
            f"Missing or extra capitals in result. Expected: {expected}, Got: {result}"
        )

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

Focuses on practical SQL skills for filtering data with WHERE, sorting results with ORDER BY, and summarizing information using GROUP BY and aggregate functions. Hands-on challenges reinforce each concept, preparing learners to analyze and summarize real-world datasets effectively.

Challenge: Sort Capitals in Descending Order

Solution