Summary  
This chapter explains how to sort retrieved data using the ORDER BY clause, which arranges results by specified columns in ascending numeric order or alphabetical string order.  

General domain of usage  
Database querying

We can sort the data we receive in a specific order. To do this, we first retrieve the data using the `SELECT` operator and then apply the `ORDER BY` clause. This clause takes the **names of columns** based on which the output will be sorted. 

Here's the syntax to help us understand:

```sql
SELECT columns
FROM table
ORDER BY column_name
```

Here is the example where we ordering the result by `continent` column:

SELECT continent 
FROM country 
ORDER BY continent;

It's important to note that when you specify a column with integer values, the sorting will be done in **ascending order**. For string columns, the sorting will follow **alphabetical order**.

Also, the `ORDER BY` clause should be placed just before the `LIMIT` clause, if a `LIMIT` clause is included in the query.

**Explanation:** In the example, you can observe that the `ORDER BY` clause sorts the data based on the `continent` column.


import unittest
import pandas as pd
import psycopg2

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 test_correct_output_sorted(self):
        """Check if capitals are selected and sorted alphabetically."""
        with open("query.sql", "r") as file:
            query = file.read()
        self.cursor.execute(query)
        rows = self.cursor.fetchall()
        capitals = [row[0] for row in rows]
        expected_capitals = [
            'Amsterdam', 'Asunción', 'Bangkok', 'Kyiv', 'Lisbon', 'Manila',
            'Mexico City', 'Monaco', 'Oslo', 'Riga', 'Stockholm', 'Tokyo',
            'Valletta', 'Washington', 'Wellington'
        ]
        capitals_no_spaces = [c.strip() for c in capitals]
        expected_capitals_no_spaces = [c.strip() for c in expected_capitals]
        is_sorted = capitals_no_spaces == sorted(expected_capitals_no_spaces)
        _dynamic_test(
            self,
            is_sorted,
            "Capitals are returned in alphabetical order.",
            f"Capitals are not sorted alphabetically. Got: {capitals_no_spaces}"
        )

    def test_only_capital_column_selected(self):
        """Check that only the 'capital' 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]
        _dynamic_test(
            self,
            columns == ['capital'],
            "Only 'capital' column is selected.",
            f"Unexpected columns selected: {columns}"
        )

    def test_no_syntax_error(self):
        """Check that query does not raise a syntax error."""
        with open("query.sql", "r") as file:
            query = file.read()
        try:
            self.cursor.execute(query)
            _dynamic_test(
                self,
                True,
                "Query executes without syntax error.",
                ""
            )
        except Exception as e:
            _dynamic_test(
                self,
                False,
                "",
                f"Query raised an error: {e}"
            )

    def test_table_and_column_exist(self):
        """Check that 'country' table and 'capital' column are referenced."""
        with open("query.sql", "r") as file:
            query = file.read().lower()
        has_country = 'from country' in query
        has_capital = 'select capital' in query
        _dynamic_test(
            self,
            has_country and has_capital,
            "Query references 'country' table and 'capital' column.",
            "Query must reference both 'country' table and 'capital' column."
        )

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)

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


test_solution.py

This course is perfect for beginners ready to explore the world of SQL. Whether you're just starting out in database management or aiming to use SQL for your application development projects, this course covers the essentials. You'll quickly learn how to leverage the full potential of SQL, from querying and managing data to seamlessly integrating it into real-world applications. By the end of the course, you'll have the confidence and skills to solve practical problems with SQL and enhance your development process.

Sorting Data

解答