Welcome to the forums.
The reason PostgreSQL server is not on the SLED repos (I’m taking your
word for it) is because SLED is made for desktops, and SLES is the server
equivalent (D = Desktop, S = Server, of course). With that said, both
versions of SLE are built from the same stuff, so many of the packages can
be intermingled, sans support, fairly safely.
Repositories are simply stores of RPMs and metadata describing them and
their versions, descriptions, what they provide I think too, etc. When
you “subscribe” to a repo you basically let Yast/zypper know about a store
of RPMs, and then those can be used. Yast/zypper have some small concept
of SLES vs. SLED vs. openSUSE, but it’s very small; pretend it doesn’t
exist, and really it doesn’t other than in the form of packages and their
dependencies.
Now factor into this the openSUSE Builds Service (OBS); this is a big set
of servers, free for use by basically anybody, to host software that can
be built for a variety of distributions of Linux (SUSE, Debian, RedHat,
etc.). Inside OBS there are packages in repositories for packages for
each of these distros, so if you happen to find PostgreSQL Server as a
package in a SLES, or SLED, or openSUSE repository, it might just work.
If you find it in a Debian repository that will probably fail because your
SLED box doesn’t know much about .deb packages, just RPM packages. Will
the SLES/openSUSE packages work for sure? Who knows… well, you will
shortly. They obviously aren’t tested to work that way, but they often
work well enough.
Personally, if I were you, I’d just get the packages for SLES 11 SP2,
match the architecture (x86_32, x86_64, etc.), and then install those
directly. If you want to avoid dependency nightmares download the entire
SLES 11 SP2 DVD and drop that in and, via Yast, add it as a repository
(remember, a repo is just a store of RPMs, whether that’s in the
filesystem, online, or on an optical disc like a DVD). Once done you can
use zypper to install anything from any connected repository:
sudo zypper in postgresql-server
I hope this helps a little; if not, ask follow-up questions.
Good luck.