Summary  
Demonstrates how to use an aggregate function to retrieve the highest value from a column using MAX().

General domain of usage  
Database querying

`MAX()` - This function returns the highest value in a particular column. Let's take a look at the following example:

Here `MAX()` returns the largest population value in the `country` table.

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_returns_largest_surfacearea(self):
        """Check that the query returns the correct largest surfacearea value."""
        with open("query.sql", "r") as file:
            query = file.read()
        self.cursor.execute(query)
        user_result = self.cursor.fetchone()[0]
        # Compute expected max value
        self.cursor.execute("SELECT MAX(surfacearea) FROM country;")
        expected = self.cursor.fetchone()[0]
        condition = user_result == expected and expected == 9363520
        _dynamic_test(
            self,
            condition,
            "Query returns the correct largest surfacearea value (9363520).",
            f"Query did not return the correct largest surfacearea. Expected 9363520, got {user_result}."
        )

    def test_query_single_column(self):
        """Check that the query returns only one column (the aggregate)."""
        with open("query.sql", "r") as file:
            query = file.read()
        self.cursor.execute(query)
        columns = [desc[0] for desc in self.cursor.description]
        condition = len(columns) == 1
        _dynamic_test(
            self,
            condition,
            "Query returns exactly one column.",
            f"Query returned {len(columns)} columns instead of 1."
        )

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.

The MAX() Function

Lösung