Question

Is it possible to make a join in mysql so that the values from the right table are returned if and only if the corresponding values in the left table don’t exist?

For example

SELECT * FROM
    (SELECT userid,status FROM date_status WHERE date='20-03-2012') ds
JOIN 
    (SELECT userid,status FROM default_weekday_status WHERE day='tuesday') dws
ON
    ds.userid=dws.userid

I would like this to return date_status.status if the row in date_status exists, but default_weekday_status.status otherwise.

(Note that 20-03-2012 is a Tuesday and no, tuesday wouldn’t be stored as a string 😉 )

Answer 1

The easiest way to do this is with the COALESCE function. I will return the first not-null value in the list of parameters.

SELECT coalesce(ds.status,dws.status) as status FROM
    date_status ds
RIGHT JOIN 
    default_weekday_status dws
ON
    ds.userid=dws.userid
WHERE
    ds.date='20-03-2012' or dws.day='tuesday'

The sub-queries have been removed, as they aren’t very efficient.

MySQL – COALESCE