MySQL: Is it best practice to check whether some column is NULL by `NOT ISNULL(column_name)`? Given the following schema, my question is that since I have already added the NOT NULL to IsBanned, would I have to check whether it is not null using the syntax NOT ISNULL(IsBanned)? And what's the best practice to do the same if I didn't add NOT NULL?
CREATE TABLE Viewer (
    AccountID varchar(25),
    IsBanned boolean                NOT NULL
                                    DEFAULT 0,
    ...
    CHECK                           (NOT ISNULL(isBanned)),
);
How many different values do you need?  It sounds like you need only &quot;true&quot; and &quot;false&quot;.  If so, declare the column to be NOT NULL.  There is no need for a NULL check if it is declared NOT NULL.  The NOT NULL performs that CHECK.
If you need yes/no/maybe or yes/no/don't-know-yet, etc, then you could declare it to be NULL (which simply means that it has an extra possible setting, namely NULL).