Question

Related to my previous post here, I have the following SELECT:

SELECT tc.[Time],tc.[From], tc.[To], tc.[Cost], tc.[Length], tc.[Type], tc.[PlaceCalled]    
FROM
TelstraCall as tc 
WHERE 
[AccountNumber] IN (@AccountNumber)
ORDER BY [Time] DESC

I’m trying to get the [Username] out of [Resource] given that the [PhoneNum] in [rtc] matches either [From] or [To], and Hogan has kindly helped me out with the first half :

USE [rtc]
SELECT [Username]
FROM [dbo].[Resource] R
JOIN ResourcePhone RP on R.ResourceId = RP.ResourceId
WHERE RP.PhoneNum = tc.[From]

Now I’m trying to work out the syntax of how to get a ‘User1’ given that [From] matches the [PhoneNum] in [rtc] and a ‘User2’ if [To] matches [PhoneNum] instead, because I can’t have them being jumbled up.

Answer 1

What you’re wanting to do is join on the same table twice to get related values based on two different references.

For this, you use table aliases. Here’s a simple example

SELECT u1.[Username] AS User1, u2.[Username] AS User2
FROM TelstraCall tc
INNER JOIN ResourcePhone rp1 ON tc.[From] = rp1.PhoneNum
INNER JOIN Resource u1 ON rp1.ResourceId = u1.Id -- guessing at column names here
INNER JOIN ResourcePhone rp2 ON tc.[To] = rp2.PhoneNum
INNER JOIN Resource u2 ON rp2.ResourceId = u2.Id