How to check if such USERNAME exists in sqlite3?

Question

Python

There is a table format:

"""CREATE TABLE IF NOT EXISTS users(
    id_records INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
    id_chat INT, 
    id_user INT,
    username TEXT,
    having_ban INT DEFAULT 0 NOT NULL, 
    having_kick INT DEFAULT 0 NOT NULL);"""

User enters username, how to check if such username exists in table? If there is, do something.
And how, having only one data, to get others? For example, I only have id_user - how can I get username and the rest from this data?

Thanks in advance!