I have a MySQL database, with a table called "posts", and two fields of my interest are "userid" and "source". I am trying to create an SQL command, which would insert a text string "http://domain-name.com" to all source fields that are empty, and that belong to user id, let's say 55.

So I need something like this:

Insert text string "domain-name.com" to table named "posts", into field called "source" when "userid" is "55"

(dont insert noting if the url, or something else is already there)

There is going to be more empty fields as the time goes by, and I am going to be adding the same url on a regular basic. So most of the fields will be already populated, but there are going to be new ones, and I am gonna have to enter the url there (let's say every week or two etc; only to the empty ones, I gues, or whichever way it is suppose to work)

Can somebody help me with that?

Thank you.