How would I do this conditional join in MS SQL? - sql

Question

I searched around for conditional joins, but it seems like they're trying to do something different that what I'm trying. I'll simplify the problem a bit for the question.
I have three tables: Users, Clients, and Employees.
The Users table has two columns, AssociatedID and UserType.
UserType is either "Client" or "Employee," and AssociatedID is a corresponding ID in either the Clients or the Employees table.
The problem is, Clients and Employees both use integers for IDs, so each table could have the same ID for something completely different (as opposed to GUIDs, which would avoid this problem).
What I want is a query that will look at a row in Users and say if UserType is Employee, then JOIN AssociatedID on the Employees table's ID, and if UserType is Client, then JOIN AssocaitedID on the Clients table's ID. So if Clients and Employees both have a Name column, I could get all of the names that go with each User, regardless of if they're a Client or an Employee. I'm using SQL Server 2008 R2 if that makes a difference here.

Answer 1

Try this:
SELECT
COALESCE(c.Name, e.Name) AssociatedName,
...
FROM Users u
LEFT JOIN Clients c ON u.AssociatedID = c.ClientId
AND u.UserType = 'Client'
LEFT JOIN Employees e ON u.AssociatedID = e.EmployeeId
AND u.UserType = 'Employee';
SQL Fiddle demo
Or: As #Tikkes pointed out, you can do this with UNION like this:
SELECT c.Name AssociatedName
FROM Users u
INNER JOIN Clients c ON u.AssociatedID = c.ClientId
AND u.UserType = 'Client'
UNION ALL
SELECT e.Name FROM Users u
INNER JOIN Employees e ON u.AssociatedID = e.EmployeeId
AND u.UserType = 'Employee';
Updated SQL Fiddle Demo
Update: For this you have to use the CASE expression like this:
SELECT
u.AssociatedID,
CASE u.UserType
WHEN 'Client' THEN c.Name
ELSE e.Name
END AS AssociationName
FROM Users u
LEFT JOIN Clients c ON u.AssociatedID = c.ClientId
LEFT JOIN Employees e ON u.AssociatedID = e.EmployeeId;
Updated SQL Fiddle Demo using CASE

Answer 2

Same Senario: I have CustomerExt have SoldTo and ShipTo Code,
and if ShipTo column has value then use ShipToName in table ShipTo otherwise use CustomerName in Customer table
Select COALESCE(B.CustomerName, C.ShiptoName) as CustomerName
FROM CustomerExt A
left join Customer B ON A.SoldTo = B.CustomerCode
AND isnull(A.ShipTo,'') =''
left join ShipTo C ON A.ShipTo = C.ShipToCode
AND isnull(A.ShipTo,'') <>''