difference between Null,Blank and Zero

Zero is a number value. It is a definite with precise mathematical properties. (You can do arithmetic on it ...) NULL means the absence of any value. You can't do anything with it except test for it. Blank is ill-defined. It means different things in different contexts to different people. For example: 1)Someone could mean a zero length string value: i.e. one with no characters in it (''). 2)Someone could mean a non-zero length string value consisting of only non-printing characters (SPACE, TAB, etc). Or maybe consisting of just a single SPACE character. 3)In some contexts (where character and string are different types), someone could mean a non-printing character value. 4)Someone could even mean "anything that doesn't show up when you print or display it". The point is that "blank" does not have a single well-defined meaning. At least not in (native) English IT terminology. It is probably best to avoid it ... if you want other IT professionals to understand what you mean. (And if someone else uses the term and it is not obvious from the context, ask them to say precisely what they mean!)