Re: Impossible for a number multiplied by zero to equal zero

Mensanator wrote:

Don Del Grande wrote:

Mensanator wrote:
Actually, you have this bass-ackwards. A NULL *will* cancel a
numerical
value. The rule is any mathematical expression containing a NULL
evaluates to NULL. Thus, 1+NULL=NULL or 3*NULL=NULL. The same applies
to 0, which is just another number: 0+NULL=NULL and 0*NULL=NULL.

You would think that 0 * NULL = 0, since NULL usually means "it could
be anything".

Could be, but is, in fact, unknown. Where I use it, this behaviour
is EXACTLY the correct interpretation. If I need to plot the
water table contours, I have to subtract the well DepthToWater
measurements from the surveyed TopOfCasing. If the water depth
is unknown, I WANT the calculated water table elevation to be
unknown, I do NOT want it to treat the unknown DTW as 0 because
that would result in a false calculation which is worse than
a missing calculation.

As it should be; a known value minus (or plus, for that matter) NULL
should be NULL, since you wouldn't know what the result would be.
However, if you know that X is a real number, then you know that zero
multiplied by X equals zero, which is where I got "0 * NULL = 0".

-- Don
.

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