Thank you

[perry]

Vanderghast and Allen, thank you for your help.

I was able to get both of your solutions to work. I was
able to see what Allen was trying to get me to do, but I
was wondering if someone could please explain what the IMP
in the following rule means:

(Publisher Is Null) IMP (PDate Is Null)

(I looked up Access 2002 help and a couple of books but
could not find it.)

 

[Allen Browne]

IMP is a logical operator. Like an "If... Then", it says that when one thing
is true, another must be also, e.g.:
IF it's raining THEN the ground is wet.
or
Rain IMP wet-ground.

The reverse does NOT follow. People often make this mistake, but you cannot
prove this statement from the above one:
Wet-ground IMP rain
That is not a valid conclusion! There may be lots of reasons why the ground
is wet (dew, water-truck, burst pipe), and to try to argue the "If ... Then"
backwards is faulty logic.

So, are the following observations consistent with our original statement?
It's raining Ground is wet. Yes, consistent.
It's raining Ground not wet No, not consistent.
Not raining Ground is wet Yes, consistent.
Not raining Ground not wet. Yes, consistent.

The 4 cases above can be represented like this:
True True True
True False False
False True True
False True True

That's the truth tree for the IMP operator. It's an abbreviation of the one
given in Help. To see the full tree (including the Nulls), open the
immediate window (ctrt+G), type IMP and then press F1.