How to return default value from SQL query

Question

Is there any easy way to return single scalar or default value if query doesn't return any row?

At this moment I have something like this code example:

IF (EXISTS (SELECT * FROM Users WHERE Id = @UserId))  
    SELECT Name FROM Users WHERE Id = @UserId  
ELSE
    --default value
    SELECT 'John Doe'

How to do that in better way without using IF-ELSE?

user_383868 · Answer

If the query is supposed to return at most one row, use a union with the default value then a limit:

SELECT * FROM 
 (
  SELECT Name FROM Users WHERE Id = @UserId`
  UNION
  SELECT 'John Doe' AS Name --Default value
 ) AS subquery
LIMIT 1

Both the query and default can have as many columns as you wish, and you do not need to guarantee they are not null.