Using Order By with Distinct on a Join (PLSQL) - sql

Question

I have written a join on some tables and I have ordered the data using two levels of ordering - one of which is the primary key of one table.
Now, with this data sorted I want to then exclude any duplicates from my data using an in-line view and the DISTINCT clause - and this is where I am coming unstuck.
I seem to be able to either sort the data OR distinct it, but never both at the same time. Is there a way around this or have I stumbled upon the SQL equivalent of the uncertainty principle?
This code returns the data sorted, but with duplicates
SELECT
ada.source_tab source_tab
, ada.source_col source_col
, ada.source_value source_value
, ada.ada_id ada_id
FROM
are_aud_data ada
, are_aud_exec_checks aec
, are_audit_elements ael
WHERE
aec.aec_id = ada.aec_id
AND ael.ano_id = aec.ano_id
AND aec.acn_id = 123456
AND ael.ael_type = 1
ORDER BY
CASE
WHEN source_tab = 'Tab type 1' THEN 1
WHEN source_tab = 'Tab type 2' THEN 2
ELSE 3
END
,ada.ada_id ASC;
This code removes the duplicates, but I lose the order...
SELECT DISTINCT source_tab, source_col, source_value FROM (
SELECT
ada.source_tab
, ada.source_col source_col
, ada.source_value source_value
, ada.ada_id ada_id
FROM
are_aud_data ada
, are_aud_exec_checks aec
, are_audit_elements ael
WHERE
aec.aec_id = ada.aec_id
AND ael.ano_id = aec.ano_id
AND aec.acn_id = 123456
AND ael.ael_type = 1
ORDER BY
CASE
WHEN source_tab = 'Tab type 1' THEN 1
WHEN source_tab = 'Tab type 2' THEN 2
ELSE 3
END
,ada.ada_id ASC
)
;
If I try and include 'ORDER BY ada_id' at the end of the outer select, I get the error message 'ORA-01791: not a SELECTed expression' which is infuriating me!!

Answer 1

Why don't you include ada_id at the selected fields of the outer query?

Answer 2

;WITH CTE AS
(
SELECT
ada.source_tab source_tab
, ada.source_col source_col
, ada.source_value source_value
, ada.ada_id ada_id
, ROW_NUMBER() OVER (PARTITION BY [COLUMNS_YOU_WANT TO BE DISTINCT]
ORDER BY [your_columns]) rn
FROM
are_aud_data ada
, are_aud_exec_checks aec
, are_audit_elements ael
WHERE
aec.aec_id = ada.aec_id
AND ael.ano_id = aec.ano_id
AND aec.acn_id = 356441
AND ael.ael_type = 1
ORDER BY
CASE
WHEN source_tab = 'Licensed Inventory' THEN 1
WHEN source_tab = 'CMDB' THEN 2
ELSE 3
END
,ada.ada_id ASC
)
select * from CTE WHERE rn<2

Answer 3

it seems that the ada_id is meaningless in the outer query.
you have removed all those values to boil it down to the distinct source_tab and source_col...
what would you expect the order to be?
you want maybe the minimum ada_id for each table and column set to be the driver for the order - (although the table name seems appropriate to me)
include the minimum ada_id in the inner query (you'll need a group by clause)
then reference that in the outer query and sort on it.