How do I limit a LEFT JOIN to the 1st result in SQ

Question

I have a bit of SQL that is almost doing what I want it to do. I'm working with three tables, a Users, UserPhoneNumbers and UserPhoneNumberTypes. I'm trying to get a list of users with their phone numbers for an export.

The database itself is old and has some integrity issues. My issue is that there should only ever be 1 type of each phone number in the database but thats not the case. When I run this I get multi-line results for each person if they contain, for example, two "Home" numbers.

How can I modify the SQL to take the first phone number listed and ignore the remaining numbers? I'm in SQL Server and I know about the TOP statement. But if I add 'TOP 1' to the LEFT JOIN select statement its just giving me the 1st entry in the database, not the 1st entry for each User.

This is for SQL Server 2000.

Thanks,

SELECT  Users.UserID, 
  Users.FirstName, Users.LastName,
  HomePhone, WorkPhone, FaxNumber

FROM Users

LEFT JOIN
 (SELECT UserID, PhoneNumber AS HomePhone
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Home') AS tmpHomePhone
 ON tmpHomePhone.UserID = Users.UserID
LEFT JOIN
 (SELECT UserID, PhoneNumber AS WorkPhone
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Work') AS tmpWorkPhone
 ON tmpWorkPhone.UserID = Users.UserID
LEFT JOIN
 (SELECT UserID, PhoneNumber AS FaxNumber
 FROM UserPhoneNumbers LEFT JOIN UserPhoneNumberTypes ON UserPhoneNumbers.UserPhoneNumberTypeID=UserPhoneNumberTypes.UserPhoneNumberTypeID
 WHERE UserPhoneNumberTypes.PhoneNumberType='Fax') AS tmpFaxNumber
 ON tmpFaxNumber.UserID = Users.UserID

Answer 1

I assume you have some primary key field on each joined table, since UserID is not unique. I'll assume your primary key is called ID. We'll take the records with the lowest ID. This meets your "first" criteria.

SELECT  Users.UserID, Users.FirstName, Users.LastName, hp.HomePhone,
        wp.WorkPhone, fn.FaxNumber
FROM Users
LEFT JOIN HomePhone hp ON hp.UserID = Users.UserID
LEFT JOIN HomePhone hp2 ON hp2.UserID = Users.UserID AND hp2.ID < hp.ID
LEFT JOIN WorkPhone wp ON wp.UserID = Users.UserID
LEFT JOIN WorkPhone wp2 ON wp2.UserID = Users.UserID AND wp2.ID < wp.ID
LEFT JOIN FaxNumber fn ON fn.UserID = Users.UserID
LEFT JOIN FaxNumber fn2 ON fn2.UserID = Users.UserID AND fn2.ID < fn.ID
WHERE hp2.ID IS NULL AND wp2.ID IS NULL AND fn2.ID IS NULL

There is a whole chapter on this type of issue, called "Ambiguous Gruops", in the book SQL Antipatterns.

Answer 2

Since it's SQL Server 2000 and ranking functions are out, you could make your subquery SELECTs aggregate:

SELECT UserID, MAX(PhoneNumber) AS HomePhone FROM [...] GROUP BY UserID

iff you don't care WHICH of a user's Home numbers are returned...