I am designing a database to keep track of our network equipment. My db has 2 tables:

sites & circuits

(circuit = cable) & (site = property location)

Each circuit runs between two sites, so it should relate to two sites, yes? The circuit table has the following columns: ID_circuit (PK), circuit_number, site1, site2. I thought that I could relate my ID_site field to both site1 and site2, but I cannot. Would I be better off creating a whole other table to hold the locations of both cable ends?

2 Answers 2


Site1 and site2 can both be FKs, linked to the ID_site PK. This works on any rdbms that i'm aware of.

Example Query:

SELECT CT.site1, CT.site2, S1.site_description, S2.site_description
FROM circuit_table AS CT
INNER JOIN site_table as S1
  ON CT.site1 = S1.ID_site
INNER JOIN site_table as S2
  ON CT.site2 = S2.ID_site
  • How would I then query this to get information on both of the sites? If I use a LEFT JOIN, I only get circuit info, if I RIGHT JOIN, I only get site info, if I INNER JOIN, I don't get anything.
    – Lily Mara
    Commented Jul 12, 2014 at 0:35
  • 1
    edited answer with example query
    – ryand
    Commented Jul 12, 2014 at 4:10
  • 1
    If you think of the two sites differently e.g. one is a "source" and the other is a "destination" make sure you name the foreign key columns appropriately and put the right id_site in the right column. If you think of them symetrically be aware of this. The combination (A,B) will be the same in your mind as (B,A) but will be a different value as far as the table is concerned. You need to look out for this when writing data and queries. Commented Jul 12, 2014 at 6:08
  • I'm using MS Access (I know), so the query I ended up using is SELECT CT.cableName, S1.siteName, S2.siteName FROM (cables AS CT INNER JOIN sites AS S1 ON CT.site1 = S1.idSite) INNER JOIN sites AS S2 ON CT.site2 = S2.idSite;
    – Lily Mara
    Commented Jul 12, 2014 at 13:41

Yes, you will be better off creating a whole separate table to hold the locations of both cable ends unless you can enforce constraints during row creation. The ambiguity mentioned by Michael Green in a comment, about how one ensures that AB is recognized as identical to BA, is the critical issue.

If you are in a position to enforce an ordering constraint on the sites during row construction then the issue is less significant, and you can take the shortcut of a single table safely.

If you require the separate Site table, you will probably wish to add a bit field such as IsSource to its Key as the simplest way to ensure that each Circuit can have only 2 ends.

  • The problem with making an IsSource column is that each site could have multiple circuits going to it. I can enforce constraints, and Rydel's query works fine.
    – Lily Mara
    Commented Jul 12, 2014 at 13:41
  • @NateMara: If you are in control of the creation process then I agree - that solution is simpler. Just be aware of the issues and ambiguities involved. Commented Jul 12, 2014 at 13:54
  • This is all in an Access DB (I know it's bad; I'm just a lowly intern, have to listen to boss), so everyone using the DB will have to use my forms to fill data, and I can ensure that they only use valid info.
    – Lily Mara
    Commented Jul 12, 2014 at 14:04

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