Case when exists then select - tsql

Question

I have requirement to select the field from the table in case statement like instead of some static value.
WHEN EXISTS(SELECT c.customer_name FROM Sales.Customer AS c
WHERE c.PersonID = #BusinessEntityID)
THEN c.customer_name
How can this be achieved or is this possible . I have taken the following from msdn site. Need to tweak to fulfill my requirement.
USE AdventureWorks2008R2;
GO
CREATE FUNCTION dbo.GetContactInformation(#BusinessEntityID int)
RETURNS #retContactInformation TABLE
(
BusinessEntityID int NOT NULL,
FirstName nvarchar(50) NULL,
LastName nvarchar(50) NULL,
ContactType nvarchar(50) NULL,
PRIMARY KEY CLUSTERED (BusinessEntityID ASC)
)
AS
-- Returns the first name, last name and contact type for the specified contact.
BEGIN
DECLARE
#FirstName nvarchar(50),
#LastName nvarchar(50),
#ContactType nvarchar(50);
-- Get common contact information
SELECT
#BusinessEntityID = BusinessEntityID,
#FirstName = FirstName,
#LastName = LastName
FROM Person.Person
WHERE BusinessEntityID = #BusinessEntityID;
SET #ContactType =
CASE
-- Check for employee
WHEN EXISTS(SELECT * FROM HumanResources.Employee AS e
WHERE e.BusinessEntityID = #BusinessEntityID)
THEN 'Employee'
-- Check for vendor
WHEN EXISTS(SELECT * FROM Person.BusinessEntityContact AS bec
WHERE bec.BusinessEntityID = #BusinessEntityID)
THEN 'Vendor'
-- Check for store
WHEN EXISTS(SELECT * FROM Purchasing.Vendor AS v
WHERE v.BusinessEntityID = #BusinessEntityID)
THEN 'Store Contact'
-- Check for individual consumer
WHEN EXISTS(SELECT * FROM Sales.Customer AS c
WHERE c.PersonID = #BusinessEntityID)
THEN 'Consumer'
END;
-- Return the information to the caller
IF #BusinessEntityID IS NOT NULL
BEGIN
INSERT #retContactInformation
SELECT #BusinessEntityID, #FirstName, #LastName, #ContactType;
END;
RETURN;
END;
GO

Answer 1

No idea what the rest of your code looks like, but typically this would be:
SELECT name = COALESCE(c.customer_name, o.other_entity_name)
FROM dbo.MainEntity AS m
LEFT OUTER JOIN dbo.Customers AS c ON m.something = c.something
LEFT OUTER JOIN dbo.OtherTable AS o ON m.something = o.something;
But other than a general idea, you haven't given quite enough information to supply a complete answer.