Re: Links between rows in a table

>> It would probably be better to always have either both or neither of
>> the symmetric relationships in the table. You could make a set of
>> triggers
>> to enforce this.

Because your relation is symmetric, you should not name them "user" and
"friend".
The duplication is useless if you add a constraint : see this

create table friendship (
user_id_1 integer references ... on delete cascade,
user_id_2 integer references ... on delete cascade,

CHECK( user_id_1 < user_id_2 )
);

user_id_1 < user_id_2 means :
- a user can't be his own friend
- only one row per friend
- when you want to know if A is friend of B, no need to make two selects,
just select where user_id_1 = min(user_id_A, user_id_B) AND user_id_2 =
max(user_id_A, user_id_B)

To get the list of friends for a user, you still need the union, but that
is no real problem. Making two queries will be marginally slower than one
query on a bigger table, but youu save precious cache space, so in the end
it could be faster.