> Ah - I've got myself into the habit of always having an id column for every
> table, so that I know I can always refer to each individual row easily.
that, indeed, is the purpose of a primary key
however, the primary key need not be a surrogate key
in the case of a relationship table, the best primary key is the pair of foreign
keys
thus,
create table userdepts
( user_id integer not null
, foreign key (user_id) references users (user_id)
, dept_id integer not null
, foreign key (dept_id) references depts (dept_id)
, primary key (user_id,dept_id)
);
some database developers who declare a surrogate id as the primary key of such a
table will forget to declare an additional unique constraint on the pair of
keys, thus opening the door to the same user being assigned to the same
department more than once -- the compound natural primary key avoids the need
for this additional constraint because it is, by definition, unique
rudy
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