I agree with modi123_1 thst ISNULL() is the way to go, but a a few words of warning for you.
Note that the MySQL ISNULL function has different syntax to the MSSQL ISNULL function (maybe not applicable in your case, but might be useful to someone who finds this via Google).
Also of note is that there is no native boolean type in MySQL as of v5.5. BOOLEAN is synonymous with TINYINT(1). So, any value you want to store that is boolean, will actually be stored as 0/1, not false/true. In PHP, this isn't a real problem with most applications, but something worthy of note, espacially if you're pushing values between languages with strong typing.
This post has been edited by e_i_pi: 23 April 2012 - 04:38 AM