Should comma be used in sentences like "Hello, Michael" / "Hello Michael" ?
For example, there is a wikipedia article Hello, sailor (with comma), but it mentions two different books, a band, and two albums, all called "Hello Sailor" without a comma. The article in Grammar-Monster.com seems to suggest a comma, but it is not clear from the text whether it is necessary if "Hello (comma?) Michael" is a complete sentence.
This is entirely optional and down to personal style. Would you take a breath between the words?
Consider the difference between:
"Yes, sir" - as might be said by a shopkeeper rapidly losing patience with his customer; and
"Yes sir" - as might be said by an enthusiastic pupil.
(There's also the somewhat unique "Yessir", barked by military personnel.)
In a longer sentence, there would always be a comma before "Michael" at the end, such as: "Do you think it will rain today, Michael?"
In your particular case - Yes, you need to use a comma between the person's name and the greeting, though in many many cases not using the comma is also allowed =)
