Comparing values in two rows and getting ORA-00918: column ambiguously defined - sql

Question

I am trying to compare values from one column in two different rows using CASE statement. Inner SELECT statements are identical and working OK giving me fields that I need. Then I inner joined them on one of the key fields (KYCID). That's where I get "column ambiguously defined" error. I tried to remove duplicate rows by using DISTINCT - still the same error.
SELECT DISTINCT e.KYCID,
CASE WHEN e.HRAC_FLAG <> f.HRAC_FLAG THEN 'FALSE' ELSE 'TRUE' END AS FLAG_COMPARISON
FROM
(SELECT gt.task_id, gt.work_item_status, gt.work_item_type, gt.component_id, xref.HRAC_FLAG,
xref.case_nbr, xref.task_id, d.kycid, d.core_component_state
FROM kyc_gbl_main.global_task gt
inner join KYC_RGN_NAM_MAIN.CASE_HRAC_XREF xref on gt.component_id = xref.component_id
inner join kyc_rgn_nam_main.account a on xref.accountid = a.accountid
inner join kyc_rgn_nam_main.country_appx_account_xref b on a.accountid = b.accountid
inner join kyc_rgn_nam_main.country_appx c on c.cntry_appx_id = b.cntry_appx_id and
c.country_appx_state = b.country_appx_state
inner join kyc_rgn_nam_main.kyc_main d on d.kycid = c.kycid
where gt.work_item_type = 'HRAC_OVERLAY'
and gt.work_item_status = 'Completed') e
INNER JOIN
(select gt.task_id, gt.work_item_status, gt.work_item_type, gt.component_id, xref.HRAC_FLAG,
xref.case_nbr, xref.task_id, d.kycid, d.core_component_state
from kyc_gbl_main.global_task gt
inner join KYC_RGN_NAM_MAIN.CASE_HRAC_XREF xref on gt.component_id = xref.component_id
inner join kyc_rgn_nam_main.account a on xref.accountid = a.accountid
inner join kyc_rgn_nam_main.country_appx_account_xref b on a.accountid = b.accountid
inner join kyc_rgn_nam_main.country_appx c on c.cntry_appx_id = b.cntry_appx_id and
c.country_appx_state = b.country_appx_state
inner join kyc_rgn_nam_main.kyc_main d on d.kycid = c.kycid
where gt.work_item_type = 'HRAC_OVERLAY'
and gt.work_item_status = 'Completed') f
ON e.KYCID = f.KYCID
WHERE e.core_component_state ='ACTIVE'
AND f.core_component_state = 'IN_PROGRESS';

Answer 1

As determined with process of elimination, you reference the same named column in a SELECT query. To avoid this name collision, consider aliasing those particular columns.
Additionally, to avoid repetition, consider using a CTE via WITH and avoid using table aliases like (a, b, c) or (t1, t2, t3). Finally, move WHERE conditions to ON to filter before combining all data sources.
WITH sub AS (
SELECT gt.task_id AS gt_task_id -- RENAMED TO AVOID COLLISION
, gt.work_item_status
, gt.work_item_type
, gt.component_id
, xref.HRAC_FLAG
, xref.case_nbr
, xref.task_id AS x_ref_task_id -- RENAMED TO AVOID COLLISION
, k.kycid
, k.core_component_state
FROM kyc_gbl_main.global_task gt
INNER JOIN KYC_RGN_NAM_MAIN.CASE_HRAC_XREF xref
ON gt.component_id = xref.component_id
AND gt.work_item_type = 'HRAC_OVERLAY' -- MOVED FROM WHERE
AND gt.work_item_status = 'Completed' -- MOVED FROM WHERE
INNER JOIN kyc_rgn_nam_main.account acc
ON xref.accountid = acc.accountid
INNER JOIN kyc_rgn_nam_main.country_appx_account_xref cax
ON acc.accountid = cax.accountid
INNER JOIN kyc_rgn_nam_main.country_appx ca
ON ca.cntry_appx_id = cax.cntry_appx_id
AND ca.country_appx_state = cax.country_appx_state
INNER JOIN kyc_rgn_nam_main.kyc_main k
ON k.kycid = ca.kycid
)
SELECT DISTINCT
e.KYCID
, CASE
WHEN e.HRAC_FLAG <> f.HRAC_FLAG
THEN 'FALSE'
ELSE 'TRUE'
END AS FLAG_COMPARISON
FROM sub e
INNER JOIN sub f
ON e.KYCID = f.KYCID
AND e.core_component_state = 'ACTIVE' -- MOVED FROM WHERE
AND f.core_component_state = 'IN_PROGRESS' -- MOVED FROM WHERE