ORACLE GROUP BY error - sql

Question

My SQL query is the following:
SELECT
oh.ORDER_NUMBER, oh.CREATED_TS, stat.STATE, stat.TIMESTAMP
FROM
IMPL_OH_ORDER_HEADER oh,
SNCR_ORDER_DISPOSITION stat,
SNCR_ORDER_DISP_HEAD stathead,
IMPL_CUST_ACCOUNT cust
WHERE
stat.DISP_TRANSACTION_ID=stathead.DISP_TRANSACTION_ID
and stathead.TRANSACTION_ID=oh.TRANSACTION_ID
and oh.ACCOUNT_ID=cust.ACCOUNT_ID
and stat.CATEGORY=100
and oh.USER_ID='a'
and cust.ACCOUNT_NUM='123'
GROUP BY oh.ORDER_NUMBER
ORDER BY stat.TRX_SEQ DESC;
It is giving the following error: ORA-00979: not a GROUP BY expression
If I remove the GROUP BY expression, I don't get the error.
What is correct GROUP BY syntax?
Research: A google search says that, if AGGREGATE functions are used, I should have all fields not used by aggregate function in the GROUP BY clause. So I tried to modify the GROUP BY expression to GROUP BY oh.ORDER_NUMBER, oh.CREATED_TS, stat.TIMESTAMP, stat.STATE.
Yet I am getting the same error.
Please help.

Answer 1

You get the error because you have STAT.TRX_SEQ in the order by list.
If you use the group by clause together with the order by clause, you must either order by a column on which you grouped or on an aggregated value.
This rule also applies to the selected values: they're either part of the group by or aggregate functions (or pseudo columns).

Answer 2

Can you try this one and see??
SELECT OH.ORDER_NUMBER,
OH.CREATED_TS,
STAT.STATE,
STAT.TIMESTAMP
FROM IMPL_OH_ORDER_HEADER OH,
SNCR_ORDER_DISPOSITION STAT,
SNCR_ORDER_DISP_HEAD STATHEAD,
IMPL_CUST_ACCOUNT CUST
WHERE STAT.DISP_TRANSACTION_ID = STATHEAD.DISP_TRANSACTION_ID
AND STATHEAD.TRANSACTION_ID = OH.TRANSACTION_ID
AND OH.ACCOUNT_ID = CUST.ACCOUNT_ID
AND STAT.CATEGORY = 100
AND OH.USER_ID = 'A'
AND CUST.ACCOUNT_NUM = '123'
GROUP BY OH.ORDER_NUMBER,
OH.CREATED_TS,
STAT.STATE,
STAT.TIMESTAMP
ORDER BY STAT.TRX_SEQ DESC;