Summary  
This chapter explains how to use the BETWEEN operator to filter query results for values that fall within a specified start and end range.  

General domain of usage  
Database querying

To check for a range of values, we should use the `BETWEEN` operator. This operator requires two values: the **start-value** and the **end-value** of the range. Also, the `AND` keyword should be placed **between** these values.

Here is the syntax for using the `BETWEEN` operator: 

```sql
SELECT columns 
FROM table 
WHERE column BETWEEN value_from AND value_to;
```

Let's see an example:

SELECT name, population
FROM country
WHERE population BETWEEN 100000 AND 3000000;

import unittest
import psycopg2
import pandas as pd

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_columns_selected(self):
        """Check that only name, capital, and population columns are selected and in correct order."""
        with open("query.sql", "r") as file:
            query = file.read()
        self.cursor.execute(query)
        columns = [desc[0] for desc in self.cursor.description]
        expected_columns = ["name", "capital", "population"]
        _dynamic_test(
            self,
            columns == expected_columns,
            "Correct columns are selected in the correct order.",
            f"Columns selected are {columns}, expected {expected_columns}."
        )

    def test_between_operator_and_values(self):
        """Check that the query uses BETWEEN with correct values (6000000 and 80000000)."""
        with open("query.sql", "r") as file:
            query = file.read().lower().replace("\\n", " ")
        has_between = "between" in query
        has_6000000 = "6000000" in query
        has_80000000 = "80000000" in query
        condition = has_between and has_6000000 and has_80000000
        _dynamic_test(
            self,
            condition,
            "BETWEEN operator with correct values is used.",
            "Query must use BETWEEN operator with 6000000 and 80000000."
        )

    def test_order_by_capital_ascending(self):
        """Check that the result is ordered by capital ascending."""
        with open("query.sql", "r") as file:
            query = file.read()
        self.cursor.execute(query)
        rows = self.cursor.fetchall()
        if not rows:
            _dynamic_test(
                self,
                False,
                "Result is ordered by capital ascending.",
                "Query result is empty, cannot check ordering."
            )
            return
        capitals = [row[1] for row in rows]
        sorted_capitals = sorted([c for c in capitals if c is not None])
        # Place None at the end if present
        if any(c is None for c in capitals):
            sorted_capitals += [None] * capitals.count(None)
        condition = capitals == sorted_capitals
        _dynamic_test(
            self,
            condition,
            "Result is ordered by capital ascending.",
            f"Result is not ordered by capital ascending: {capitals}"
        )

    def test_correct_rows_selected(self):
        """Check that only countries with population between 6000000 and 80000000 are returned."""
        with open("query.sql", "r") as file:
            query = file.read()
        self.cursor.execute(query)
        rows = self.cursor.fetchall()
        # The population column is at index 2
        for row in rows:
            population = row[2]
            condition = population is not None and 6000000 <= population <= 80000000
            if not condition:
                _dynamic_test(
                    self,
                    False,
                    "All rows have population between 6000000 and 80000000.",
                    f"Row with population {population} is not in the required range."
                )
                return
        _dynamic_test(
            self,
            True,
            "All rows have population between 6000000 and 80000000.",
            "Some rows have population outside the required range."
        )

    def test_query_syntax(self):
        """Check that the query executes without syntax errors."""
        with open("query.sql", "r") as file:
            query = file.read()
        try:
            self.cursor.execute(query)
            _dynamic_test(
                self,
                True,
                "Query executes without syntax errors.",
                "Query has a syntax error."
            )
        except Exception as e:
            _dynamic_test(
                self,
                False,
                "Query executes without syntax errors.",
                f"Query has a syntax error: {e}"
            )

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

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.

Checking for a Range of Values

解答