What is the difference between `join`, `outer join`, `exists join`, and `notexists join` in X++?

Use the join type based on what you actually need back.

• join — only return records where both sides match.
• outer join — return the main record even when the joined record does not exist.
• exists join — only care whether a matching record exists.
• notexists join — only return records where no matching record exists.

If you do not need fields from the second table and only care whether a row exists, exists join is usually the cleanest option.