PostgreSQL has a variety of sizes of integer types, from smallint (2 bytes)
to integer (4 bytes) to bigint (8 bytes), as well as other numeric
types.
It does not however support unsigned versions of these numeric types.
That means, with an integer for instance, we can store numbers between
-2147483648 and +2147483647. That's everything that can fit into 4 bytes.
In a system that supported 4 byte unsigned integers we'd be able to represent
from 0 all the way up to 4294967295.
In PostgreSQL, we're limited to these signed numeric types.
That means if we were hoping that the data type could essentially enforce a
non-negative restriction on the data in one of our columns, we're going to have
to be more creative. The obvious choice to me is to consider adding a check
constraint
(e.g. quantity integer check (quantity > 0)).
Another option, as pointed out by this StackOverflow answer, is to create a user-defined domain type that restricts valid values. To me, the ergonomics of using a domain type are a bit awkward and not worth the effort.
With either of these solutions, we are only approximating an unsigned integer and do not actually have the same range of values available.