Calculation that returns incomprehensible values

[caifa]

I have a field, let's call it Field D, that is the result of a calculation.
field D = field A + Field B - Field C

sometimes the calculation works, but in some cases it returns incomprehensible values, i.e. -8,8817842E-16 (whereas I expected to find 0)

 

[drlex]

It's all due to the wonderful ways which computers (and hence databases) store decimal numbers using binary methods. Just use ROUND or formatting where necessary to remove or mask this problem.

soi la, soi carré