class SQLGenerator:
    """
    This class generates SQL statements for common operations on a table, such as SELECT, INSERT, UPDATE, and DELETE.
    """
    def __init__(self, table_name):
        self.table_name = table_name

    def insert(self, data):
        """
        Generates an INSERT SQL statement based on the given data.
        :param data: dict. The data to be inserted, in dictionary form where keys are field names and values are field values.
        :return: str. The generated SQL statement.
        >>> sql.insert({'key1': 'value1', 'key2': 'value2'})
        "INSERT INTO table1 (key1, key2) VALUES ('value1', 'value2');"
        """

        fields = ", ".join(data.keys())
        values = ", ".join([f"'{value}'" for value in data.values()])
        sql = f"INSERT INTO {self.table_name} ({fields}) VALUES ({values})"
        return sql + ";"

    def update(self, data, condition):
        """
        Generates an UPDATE SQL statement based on the given data and condition.
        :param data: dict. The data to be updated, in dictionary form where keys are field names and values are new field values.
        :param condition: str. The condition expression for the update.
        :return: str. The generated SQL statement.
        >>> sql.update({'field1': 'new_value1', 'field2': 'new_value2'}, "field3 = value1")
        "UPDATE table1 SET field1 = 'new_value1', field2 = 'new_value2' WHERE field3 = value1;"
        """

        set_clause = ", ".join([f"{field} = '{value}'" for field, value in data.items()])
        sql = f"UPDATE {self.table_name} SET {set_clause} WHERE {condition}"
        return sql + ";"

    def delete(self, condition):
        """
        Generates a DELETE SQL statement based on the given condition.
        :param condition: str. The condition expression for the delete.
        :return: str. The generated SQL statement.
        >>> sql.delete("field1 = value1")
        'DELETE FROM table1 WHERE field1 = value1;'
        """

        sql = f"DELETE FROM {self.table_name} WHERE {condition}"
        return sql + ";"

    def select_female_under_age(self, age):
        """
        Generates a SQL statement to select females under a specified age.
        :param age: int. The specified age.
        :return: str. The generated SQL statement.
        >>> sql.select_female_under_age(30)
        "SELECT * FROM table1 WHERE age < 30 AND gender = 'female';"
        """

        condition = f"age < {age} AND gender = 'female'"
        return self.select(condition=condition)

    def select_by_age_range(self, min_age, max_age):
        """
        Generates a SQL statement to select records within a specified age range.
        :param min_age: int. The minimum age.
        :param max_age: int. The maximum age.
        :return: str. The generated SQL statement.
        >>> sql.select_by_age_range(20, 30)
        'SELECT * FROM table1 WHERE age BETWEEN 20 AND 30;'
        """

        condition = f"age BETWEEN {min_age} AND {max_age}"
        return self.select(condition=condition)

Method:
def select(self, fields=None, condition=None):
        if fields is None:
            fields = "*"
        else:
            fields = ", ".join(fields)
        sql = f"SELECT {fields} FROM {self.table_name}"
        if condition is not None:
            sql += f" WHERE {condition}"
        return sql + ";"

Summary: Generates a SELECT SQL statement based on the specified fields and conditions.