SELECT <columns>
FROM [DatabaseA].[Sales_Forecast], [DatabaseB].[Customers]
ORDER BY [DatabaseB].[Customer_ID];

Cannot resolve the collation conflict between "Latin1_General_BIN" and "SQL_Latin1_General_CP1_CI_AS" in the equal to operation.

SELECT <columns>
FROM [DatabaseA].[Sales_Forecast] sf
    JOIN [DatabaseB].[Customers] c
        ON sf.[Customer_ID] = c.[Customer_ID]
ORDER BY c.[Customer_ID];

SELECT <columns>
FROM [DatabaseA].[Sales_Forecast] sf
    JOIN [DatabaseB].[Customers] c
        ON sf.[Customer_ID] COLLATE Latin1_General_CI_AS = c.[Customer_ID] COLLATE Latin1_General_CI_AS
ORDER BY c.[Customer_ID]