Why do "yes/no" values in Microsoft Access map to -1 and 0?

The use of -1 puzzles me. I would have expected the values to be:

• NO --> 0
• YES --> +1

Is there an historical reason behind the use of -1 instead of +1?

1 Answer

As discussed on Stack Overflow here, Yes/True values appear in Access as -1 because

• Yes/No fields in Access mimic bit fields,
• Integer values in Access are signed, two's complement values,
• No/False is represented by 0, and
• the only other two's complement value of a 1-bit number is -1. To illustrate, two's complement numbers can have the following values:

3-bit:

bits integer ---- ------- 000 0 001 1 010 2 011 3 100 -4 101 -3 110 -2 111 -1 

2-bit:

bits integer ---- ------- 00 0 01 1 10 -2 11 -1 

1-bit:

bits integer ---- ------- 0 0 1 -1 

For the integer representation of a bit field (i.e., 1-bit), if 0 is No/False then the only other value available for Yes/True is -1.