一个表中的多个外键链接到第二个表中的单个主键

Question

I have a database with three tables, a household table, an adults table and a users table. The Household table contains two foreign keys, iAdult1ID and iAdult2ID. The Users table has a iUserID primary key and the Adult table has a corresponding iUserID foreign key. One of the columns in the Users table is strUsername, an e-mail address.

I am trying to write a query that will allow me to search for an e-mail address for either adult that has a relation to the household. So I have two questions, assuming that all the values are not null, how can I do this?

And two, in reality, iAdult2ID can be null, is it still possible to write a query to do this?

Thanks for your help. Let me know if you need any more information.

Answer 1

Yes, you just left join to the same table twice:

select u1.strUsername, u2.strUsername
FROM Household h
LEFT JOIN Adult a1 on a1.ID = h.iAdult1ID
LEFT JOIN Users u1 on u1.ID = a1.iUserID
LEFT JOIN Adult a2 on a2.ID = h.iAdult2ID
LEFT JOIN Users u2 ON u2.ID = a2.iUserID

Answer 2

While the accepted answer is correct I thought it would be helpful to point out that the original problem is a consequence of how the data is modeled.

Ideally you should move the AdultIds out of the HouseHold table and into a new table called HouseholdAdults which contains both the HouseholdId and the iAdultId. Then you can have one or more adults per household.

Heres a screenshot to illustrate what I mean.

---

---

Currently you have restricted the relationship between adults and household to at least one and no more than two. What about houses with no adults or three? Also, you can't add a household record before adding an adult record? Where do you store information about an adults relationship to a household? For example, who is the owner, tenant, when they moved in, bought it etc. Basically anything relevant to the relationship.

These are all probably non-issues for you at the moment but still worth some thought I think.

This remodeling then resolves the issues you have with querying the data. With the new table in place to join the household table and the adult table you can easily query the relationship with the following.

SELECT Adult.iAdultId, Household.HouseHoldId, Users.strUserName FROM Adult 
INNER JOIN HouseholdAdults ON Adult.iAdultId = HouseholdAdults.iAdultId 
INNER JOIN Household ON HouseholdAdults.HouseholdId = Household.HouseHoldId 
INNER JOIN Users ON Adult.iUserId = Users.iUserId

Heres some links explaining it further.
Many-to-Many relations
Database normalization