Use a combination of CONCAT
and IFNULL
(to handle the NULL
case):
UPDATE `table`
SET `City` = IFNULL(CONCAT(`City`, " Tokyo"), "Tokyo")
WHERE `ContactName` = "Maria Anders";
However, appending seems wrong. If you want multiple City values (a many-to-many relationship), you should have a separate table for cities and a separate table for person-city relationships. Read about database normalisation.
http://stackoverflow.com/questions/5524872/mysql-append-string