A database is a named collection of SQL objects
("database objects"). Generally, every database
object (tables, functions, etc.) belongs to one and only one
database. (But there are a few system catalogs, for example
pg_database, that belong to a whole cluster and
are accessible from each database within the cluster.) More
accurately, a database is a collection of schemas and the schemas
contain the tables, functions, etc. So the full hierarchy is:
server, database, schema, table (or some other kind of object,
such as a function).
An application that connects to the database server specifies in
its connection request the name of the database it wants to connect
to. It is not possible to access more than one database per
connection. (But an application is not restricted in the number of
connections it opens to the same or other databases.) It is
possible, however, to access more than one schema from the same
connection. Schemas are a purely logical structure and who can
access what is managed by the privilege system. Databases are
physically separated and access control is managed at the
connection level. If one EnterpriseDB server
instance is to house projects or users that should be separate and
for the most part unaware of each other, it is therefore
recommendable to put them into separate databases. If the projects
or users are interrelated and should be able to use each other's
resources they should be put in the same databases but possibly
into separate schemas. More information about managing schemas is
in Section 4.7.
Note: The SQL standard calls databases "catalogs", but there
is no difference in practice.