Ben Leslie <benno(at)benno(dot)id(dot)au> writes:
> "Technically, PRIMARY KEY is merely a combination of UNIQUE and NOT NULL"
>
> I wanted to clarify if that was, technically, true.
Yes, but see below.
> "identifying a set of columns as primary key also provides metadata
> about the design of the schema, as a primary key implies that other
> tables can rely on this set of columns as a unique identifier for
> rows."
This means that e.g. you can use
ALTER TABLE othertbl FOREIGN KEY (refid) REFERENCES mytbl
without specifying the column(s) of mytbl. This implies that there can
be only one primary key (possibly covering more than one column), and
that's the second difference to UNIQUE NOT NULL.