Alter PROCEDURE [dbo].[sp_GetCompany]
@CompanyID bigint OUTPUT
AS
SET NOCOUNT ON;
SELECT @CompanyID = t1.CompanyID + ',' 
FROM t1
WHERE EXISTS
(Select 1 
FROM t2 d
where d.CompanyID=t1.CompanyID 
union all
SELECT 1
FRPM t3 e
WHERE e.CompanyID=t1.CompanyID
)
GO

Error: A SELECT statement that assigns a value to a variable must not be combined with data-retrieval operations.

SELECT @CompanyID = CAST(@CompanyID AS varchar(20))+ ',', '' + 
[CompanyID]
FROM t1

SELECT @CompanyID = ISNULL(@CompanyID + ',', '') + [CompanyID]  
FROM t1
WHERE EXISTS
(Select 1 
FROM t2 d
where d.CompanyID=t1.CompanyID 
union all 
SELECT 1
FRPM t3 e
WHERE e.CompanyID=t1.CompanyID