Aggregate function help - mssql - sql-server

Question

Need some help getting sales by month for a customer and his agent channel
have a customer table that looks something like this
customer table:
cust_id bigint,
agent_id bigint,
name varchar(200),
customer_level int,
date_signed datetime
a customer level = 1 a customer who can transact; agent_id = null
a customer_level = 2 an agent of a level_1 customer and can transact, level_1 cust_id = level_2 agent_id
Transaction table:
tx_id bigint,
tx_date datetime,
description varchar(200),
amount money,
cust_id bigint
SQL
SELECT datepart(month, t.tx_date) AS TX_MONTH
,CASE
WHEN c.customer_level = 2
THEN a.NAME
ELSE c.NAME
END AS CUSTOMER
,count(t.amount)
,sum(t.amount)
FROM TRANSACTION t
INNER JOIN customer c ON t.cust_id = c.cust_id
LEFT JOIN customer a ON c.agent_id = a.cust_id
WHERE t.tx_date >= '2014-01-01 00:00:00.000'
GROUP BY datepart(month, t.tx_date)
ORDER BY 1 ASC
===
for starters this sql wont work need to group by c.name, a.name
however the intended result will not be realised....which is to get monthly sales by a customer and his agent channel
HELP!

Answer 1

Assumptions I'm making:
You will likely want year as part of the result set in case this report is used across the new year boundary.
You may want to pass parameters to this query, I'm going to use #CustID BIGINT, and #StartDate DATE, #EndDate DATE to help limit rows for performance reasons in the case of large result sets.
You will need to group by the expression used in the select clause.
Try this:
SELECT MONTH(t.tx_date) AS TX_MONTH, YEAR(t.tx_date) AS TX_YEAR
,CASE
WHEN c.customer_level = 2
THEN a.NAME
ELSE c.NAME
END AS CUSTOMER
,count(t.amount)
,sum(t.amount)
FROM TRANSACTION t
INNER JOIN customer c ON t.cust_id = c.cust_id
LEFT JOIN customer a ON c.agent_id = a.cust_id
WHERE CAST(t.tx_date AS DATE) BETWEEN #StartDate AND #EndDate
AND c.cust_id = #CustID
GROUP BY YEAR(t.tx_date), MONTH(t.tx_date)
,CASE
WHEN c.customer_level = 2
THEN a.NAME
ELSE c.NAME
END
ORDER BY YEAR(t.tx_date), MONTH(t.tx_date)
This should give you a result set with Year, Month of transactions the Agent name or Customer Name, for a given customer between two dates.