MySQL IF NOT NULL, then display 1, else display 0

Instead of COALESCE(a.addressid,0) AS addressexists, use CASE:

CASE WHEN a.addressid IS NOT NULL 
       THEN 1
       ELSE 0
END AS addressexists

or the simpler:

(a.addressid IS NOT NULL) AS addressexists

This works because TRUE is displayed as 1 in MySQL and FALSE as 0.