sumif help

[Wayne Schuller <k_wayne schuller id au>]

hi,

I'm trying to use sumif but I don't quite understand what conditions I
can use.

I have a table with two columns:

A       B
21      232
2       
9       343
2       2
3       
5       

I want to sum the values in A where the value in B is BLANK.

Is there a way to do this using SUMIF?

So far the closest I get is: =sumif(B1:B10, ">0", A1:A10)
This forumula gives me the opposite of what I want.
Is there a NOT operator for the conditions? (I don't think I can use the
NOT() function in a condition?)
Is there a way of checking for BLANK/NULL in a condition? (again, this
is different from asking for the BLANK() function - I need to be able to
put it in the condition argument)

I've been googling all over the place for how to do this in excel, but
no luck.

Thanks,
Wayne