On Fri, 2011-05-06 at 09:00 +0100, Rocio Gomez Escribano wrote:
> Tables "client" an "user" are quite similar, but they don't have any
> intersection, I mean, if somebody is a client, he or she cant be a user. So,
> I have his or her driving license and I need to know what kind of person is.
>
> Im trying some join left, right, but I'm unable to get it!!
>
OK, a couple of questions:
Are you absolutely sure the two sets of people are mutually exclusive?
Why use two separate "user" and "client" tables rather than one "person"
table with a typeId which foreign keys on to a personType table? Is the
output of two columns named PersonType,PersonId acceptable for this
query?
Assuming your design is correct as it stands or fixed in its current
state you can achieve what you want with a join.
select null as userID, clientID
from client
where clientCodeDrivingLicense = 321321321
UNION
select userId, null as clientID
from user
where userCodeDrivingLicense = 321321321
With regard to the performance of this system over time I'd suggest you
want a unique index on the DrivingLicense column/columns.
Hope that helps,
Nigel
