import sqlite3

class Database():
    def __init__(self, db_file):
        self.db_file = db_file
        self._create_table()

    def _create_table(self):
        with sqlite3.connect(self.db_file) as conn:
            cursor = conn.cursor()
            cursor.execute('''CREATE TABLE IF NOT EXISTS timeseries
                              (timestamp INTEGER PRIMARY KEY, value REAL)''')
            conn.commit()

    def insert_data(self, timestamp, value):
        with sqlite3.connect(self.db_file) as conn:
            cursor = conn.cursor()
            cursor.execute('''INSERT INTO timeseries (timestamp, value)
                              VALUES (?, ?)''', (timestamp, value))
            conn.commit()

    def fetch_data(self, limit):
        with sqlite3.connect(self.db_file) as conn:
            cursor = conn.cursor()
            cursor.execute('''SELECT timestamp, value FROM timeseries
                              ORDER BY timestamp DESC LIMIT ?''', (limit,))
            return cursor.fetchall()