It seems that MySQL's COALESCE allows to input NULL values as if all the values are NULL, NULL will be returned.
in reply to Re: How to get a placeholder to keep the current column value?
in thread How to get a placeholder to keep the current column value?
I quote from MySQL Manual:
Returns first non-NULL element in list:
mysql> SELECT COALESCE(NULL,1);
mysql> SELECT COALESCE(NULL,NULL,NULL);
UPDATE: ops, i wasn't careful enough to check the solution offered. In that case NULL won't be returned so Jenda's assumption is right.