SQL Create a new column with if/else condition - sql

Question

I've been working on this task for way too long and I'm still stuck. What I need to do is, working on database AdventureWorks2014, retrieve CustomerIDs and Average of orders they made (all customers of course). My problem begins when I try to insert another column (called Valuable), that is supposed to have values:
'Y' if CustomerOrders > 10*AverageOfOrders and
'N' if CustomerOrders < 10*AverageOfOrders.
I should use CTE, that's not that important though. I did it with CASE statement, yet I keep getting an error
Incorrect syntax near the keyword 'CASE'.
If anyone could explain to me what am I doing wrong, that would be amazing. Below is my code:
WITH CustID AS
(
SELECT CustomerID, COUNT(CustomerID) AS "NrOfOrdersPerCustomer"
FROM Sales.SalesOrderHeader
GROUP BY CustomerID
),
AvgNr AS
(
SELECT AVG("NrOfOrdersPerCustomer") AS "AvgNrOfOrders"
FROM CustID
),
Joint AS
(
SELECT CustID.CustomerID, 'NULL' AS "Valuable", CustID."NrOfOrdersPerCustomer", AvgNr."AvgNrOfOrders", 10*AvgNr."AvgNrOfOrders" AS "MultipliedBy10"
CASE
WHEN "NrOfOrdersPerCustomer" > "MultipliedBy10" THEN 'Y'
ELSE 'N'
FROM CustID, AvgNr
END
)
SELECT * FROM Joint;

Answer 1

There is already an answer that explains where your syntax errors are (missing comma and incorrect case expression), however, I thought I would just post an alternative solution that is much simpler:
WITH CustID AS
(
SELECT CustomerID, COUNT(CustomerID) AS NrOfOrdersPerCustomer
FROM Sales.SalesOrderHeader
GROUP BY CustomerID
)
SELECT CustomerID,
NrOfOrdersPerCustomer,
AVG(NrOfOrdersPerCustomer) OVER() AS AvgNrOfOrders,
AVG(NrOfOrdersPerCustomer) OVER() * 10 AS MultipliedBy10,
CASE WHEN NrOfOrdersPerCustomer > AVG(NrOfOrdersPerCustomer) OVER() * 10 THEN 'Y'
ELSE 'N'
END AS Valuable
FROM CustID;
This leverages the use a window function - AVG(NrOfOrdersPerCustomer) OVER() - meaning you can get the average of all customers without having to do a separate subquery. I have also removed quote marks from your aliases, for no other reason that they are not necessary unless your alias contains special characters, and I find them quite distracting, especially when not used consistently.

Answer 2

Last CTE is wrong as
Joint AS
(
SELECT CustID.CustomerID,
'NULL' AS "Valuable",
CustID."NrOfOrdersPerCustomer",
AvgNr."AvgNrOfOrders",
10*AvgNr."AvgNrOfOrders" AS "MultipliedBy10" <-- missing , here
CASE
WHEN "NrOfOrdersPerCustomer" > "MultipliedBy10" THEN 'Y'
ELSE 'N' <-- END should come here
FROM CustID, AvgNr
END <-- not here
)
Your last CTE should look like
Joint AS
(
SELECT CustID.CustomerID,
NULL AS "Valuable",
CustID."NrOfOrdersPerCustomer",
AvgNr."AvgNrOfOrders",
10*AvgNr."AvgNrOfOrders" AS "MultipliedBy10",
CASE
WHEN "NrOfOrdersPerCustomer" > "MultipliedBy10" THEN 'Y'
ELSE 'N' END AS Computed_Column
FROM CustID, AvgNr
)

Answer 3

At the end of your SELECT line after AS "MultipliedBy10", you need a comma. Also, your CASE statement is out of order at the end. END needs to be before the FROM statement. Corrected below:
WITH CustID AS
(
SELECT CustomerID, COUNT(CustomerID) AS "NrOfOrdersPerCustomer"
FROM Sales.SalesOrderHeader
GROUP BY CustomerID
),
AvgNr AS
(
SELECT AVG("NrOfOrdersPerCustomer") AS "AvgNrOfOrders"
FROM CustID
),
Joint AS
(
SELECT CustID.CustomerID, 'NULL' AS "Valuable", CustID."NrOfOrdersPerCustomer", AvgNr."AvgNrOfOrders", 10*AvgNr."AvgNrOfOrders" AS "MultipliedBy10",
CASE
WHEN "NrOfOrdersPerCustomer" > "MultipliedBy10" THEN 'Y'
ELSE 'N'
END
FROM CustID, AvgNr
)
SELECT * FROM Joint;