SQL Server 2008 - Error Order by case - sql-server-2008

Question

I'm getting the following error messages:
Msg 8120, Level 16, State 1, Line 1
Column 'customers.member_category' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
Msg 104, Level 16, State 1, Line 1
ORDER BY items must appear in the select list if the statement contains a UNION, INTERSECT or EXCEPT operator.
How can I order the results by the case?
Thanks
SELECT
Date,
order_id,
member_category,
COUNT(*) AS no_items,
SUM(item_amount) AS total_amount
FROM
order_items OI (NOLOCK) JOIN customers C (NOLOCK)
ON OI.CUSTOMER_NO = C.CUSTOMER_NO
WHERE
DATE = '01 FEB 2014'
GROUP BY
order_id,
member_category
UNION
SELECT
'',
'',
'Total',
COUNT(*) AS no_items,
SUM(item_amount) AS total_amount
FROM
order_items OI (NOLOCK) JOIN customers C (NOLOCK)
ON OI.CUSTOMER_NO = C.CUSTOMER_NO
WHERE
DATE = '01 FEB 2014'
GROUP BY Date WITH ROLLUP
ORDER BY Date ASC, CASE member_category WHEN 'VIP' THEN 1
WHEN 'STD' THEN 2
WHEN 'GLD' THEN 3
END

Answer 1

The second part of your UNION statement does not contain field member_category. This violates basic principle of UNION which is that number and order of columns in each statement should be same.
To resolve the issue, you need to add member_category both in the list of retrieved columns and in the GROUP BY clause.