Question

I’m looking at linked data in MS Access.

The “Yes/No” fields contain the value -1 for YES and 0 for NO. Can someone explain why such a counter-intuitive value is used for “Yes”? (Obviously, it should be 1 and 0)

I imagine there must be a good reason, and I would like to know it.

Answer 1

The binary representation of False is 0000000000000000 (how many bits are used depends on the implementation). If you perform a binary NOT operation on it, it will be changed to 1111111111111111, i.e. True, but this is the binary representation of the signed integer -1.

A bit of 1 at the most significant position signals a negative number for signed numbers. Changing the sign of a number happens by inverting all the bits and adding 1. This is called the Two’s complement.

Let us change the sign of 1111111111111111. First invert; we get:
0000000000000000

Then add one:
0000000000000001, this is 1.

This is the proof that 1111111111111111 was the binary representation of -1.

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UPDATE

Also, when comparing these values do not compare

x = -1

or

x = 1

instead, do compare

x <> 0

this always gives the correct result, independently of the convention used. Most implementations treat any value unequal zero as True.