Re: Restrictions for a specific situation in my DB

On Mon, May 11, 2020 at 3:33 PM JORGE MALDONADO <jorgemal1960(at)gmail(dot)com>
wrote:

> Hi,
>
> I have a table with the following structure:
>
> ------------------------------------------------------------------------------
>
> FIELD TYPE COMMENTS
>
> ------------------------------------------------------------------------------
> id serial primary key
> m1 integer id of record in another table
> m2 integer id of record in another table
>
> So, "m1" and "m2" are foreign keys.
> I have read that it is a good practice to define an index for each foreign
> key so "m1" and "m2" are also regular and independent indexes.
>
> Now, the combination of "m1" and "m2" together cannot be duplicated. So
> for example, the following entries
>
> ---------------------------------------------------------------------
> id m1 m2 COMMENTS
> ---------------------------------------------------------------------
> 1 2 10
> 2 8 3
> 3 18 1
> 4 2 10 This is invalid.
> 5 13 8 This is invalid.
> 6 18 18 This is invalid.
>

Its nice you provided examples but I'm pretty sure you have one (maybe two)
typos - one in row 2 (m2) and one in row 3 (m2)...otherwise your stated
rule and the data don't agree.

>
> Maybe the case for records 4 and 5 can be achieved by setting 2
> restrictions:
> Restriction 1: "m1 + m2" fields
> Restriction 2: "m2 + m1" fields
>
> But, is this a good approach?
>

Sure, two separate multi-column unique indexes and a row constraint that m1
!= m2 would work. Whether its "good" depends greatly on how the model/data
is going to be used.

David J.