Re: Links between rows in a table

On 2005-03-06 20:26, PFC wrote:
>     Because your relation is symmetric, you should not name them "user" and  
> "friend".

A good point, thank you.

>     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)

This is what we were planning to do on the application side, but a CHECK
constraint is even better. It will be used and enforced by those DB
engines that understand it, and ignored by the one engine that doesn't.

>     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.

Thank you for your insight. We will rename the columns, add the CHECK
and go ahead with this setup.


regards,
stefan weiss