[Mikhail_Edoshin]

As far as I understand NULLs were indeed a pragmatic choice, but the chief reason was the need to compute derived table values. E.g. in a join it is normal to get unknown values for a cell. This has to be expressed somehow. How? NULL seems to be a reasonably good generic solution to this. It may be possible to come up with a different generic solution, but such a solution would probably require something like conditional fields in a table and this is a whole new level of relational logic.

[cubefox]

I wonder how Codd's original relational algebra handled this case, as he didn't have NULL cells.

[Mikhail_Edoshin]

This part I do not know, but NULLs are either Codd’s idea or the one he sided with.

Another example when an invention goes against the established logic would be quaternions.

[at_compile_time]

>Another example when an invention goes against the established logic would be quaternions.

How so?

[bazoom42]

I don’t think it had outer joins.