SQL Server : "invalid column name" error after GROUP BY on subquery - sql

Question

I am trying to perform the following query on a SQL Server database table with a GROUP BY on a column which results from a CASE statement done on a subquery:
SELECT
AVG(sales) as avg_sales,
COUNT(*) as total_sales,
CASE
WHEN (pay_method='cc') AND (user_type='subscriber') THEN 'cc-subscribed'
WHEN (pay_method='cash') AND (user_type='subscriber') THEN 'cash-subscribed'
ELSE 'standard'
END as user_payment_type
FROM
(SELECT
column1, column2,
UserType as user_type,
CASE
WHEN column1='something' AND column2='something_else' THEN 'cc'
WHEN column1='something_else' AND column2='something' THEN 'cash'
END as pay_method
FROM MyTable) b
GROUP BY
user_payment_type
The error I am getting is
MSSQLDatabaseException: (207, b"Invalid column name 'user_payment_type'.DB-Lib error message 20018, severity 16:\nGeneral SQL Server error: Check messages from the SQL Server\n")
Note that the column name user_payment_type is unique and does not already exist in MyTable.

Answer 1

SQL Server does not allow the use of this aliased column in the group by clause (others like MySql do allow it) because the group by clause is executed before select.
You have to use that case statement:
group by CASE
WHEN (pay_method='cc') AND (user_type='subscriber') THEN 'cc-subscribed'
WHEN (pay_method='cash') AND (user_type='subscriber') THEN 'cash-subscribed'
ELSE 'standard'
END

Answer 2

Your SELECT and GROUP BY should match. You could avoid duplicating code by using CROSS APPLY:
WITH cte AS (
SELECT column1,
column2,
UserType as user_type,
CASE
WHEN column1='something' AND column2='something_else' THEN 'cc'
WHEN column1='something_else' AND column2='something' THEN 'cash'
END as pay_method
FROM MyTable
)
SELECT AVG(c.sales) as avg_sales,
COUNT(*) as total_sales,
s.user_payment_type
FROM cte c
CROSS APPLY (SELECT CASE
WHEN (pay_method='cc') AND (user_type='subscriber') THEN 'cc-subscribed'
WHEN (pay_method='cash') AND (user_type='subscriber') THEN 'cash-subscribed'
ELSE 'standard' END) s(user_payment_type)
GROUP BY s.user_payment_type

Answer 3

As others have noted, you can't reference column aliases in the group by clause, but should reference the same expression there too.
Note, however, that you're performing two calculations on the same data. You could perform both calculations in the same subquery to make the query shorter and easier to maintain:
SELECT
AVG(sales) as avg_sales,
COUNT(*) as total_sales,
user_payment_type
FROM (
SELECT sales,
CASE
WHEN column1 = 'something' AND
column2 = 'something_else' AND /* These are the conditions for cc */
user_type = 'subscriber'
THEN 'cc-subscribed'
WHEN column1 = 'something_else' AND
column2 = 'something' AND /* conditions for cash */
user_type = 'subscriber'
THEN 'cash-subscribed'
ELSE 'standard'
END as user_payment_type
FROM MyTable
) b
GROUP BY
user_payment_type

Answer 4

A simple way to do this without nested subqueries uses apply:
SELECT v1.user_payment_type,
AVG(t.sales) as avg_sales,
COUNT(*) as total_sales
FROM MyTable t CROSS APPLY
(VALUES (CASE WHEN t.column1 = 'something' AND t.column2 = 'something_else' THEN 'cc'
WHEN t.column1 = 'something_else' AND t.column2 = 'something' THEN 'cash'
END
)
) v(pay_method) CROSS APPLY
(VALUES (CASE WHEN v.pay_method = 'cc' AND t.user_type = 'subscriber' THEN 'cc-subscribed'
WHEN v.pay_method = 'cash' AND t.user_type = 'subscriber' THEN 'cash-subscribed'
ELSE 'standard'
END)
) v1(user_payment_type)
GROUP BY v1.user_payment_type;
This allows you to define interdependent definitions without nesting subqueries or CTEs or repeating definitions.