I have a problem with my SQL query that take time to get all records from database. Any body help me. Below is a sample of database:

order(order_id, order_nm)
customer(customer_id, customer_nm)
orderDetail(orderDetail_id, order_id, orderDate, customer_id, Comment)

I want to get latest customer and order detail information.
Here is may solution:

I've created a function that GetLatestOrderByCustomer(CusID) to get lastest Customer information.

CREATE FUNCTION [dbo].[GetLatestOrderByCustomer]
(
    @cus_id int
)
RETURNS varchar(255)
AS
BEGIN
    DECLARE @ResultVar varchar(255)

    SELECT @ResultVar = tmp.comment
    FROM 
    (
        SELECT TOP 1 orderDate, comment
        FROM orderDetail
        WHERE orderDetail.customer_id = @cust_id
    ) tmp


    -- Return the result of the function
    RETURN @ResultVar

END

Below is my SQL query

SELECT 
      customer.customer_id
    , customer.customer_nm
    , dbo.GetLatestOrderByCustomer(customer.customer_id)
FROM Customer
    LEFT JOIN orderDetail
        ON orderDetail.customer_id = customer.customer_id

It's take time to run the function. Could anybody suggest me any solutions to make it better?

Thanks in advance.