Main select query together with subquery having "select * dual" - oracle

Question

I have the following query that having a sub-query to select from dual. Both result of main and subquery need to be display in the output.
SELECT
a.ROW_EXISTS AS CLIENT_EXIST,
c.AP_Before AS AP_before,
c.AP_TIMESTAMP AS AP_TIMESTAMP,
cd.AAM_FLAG AS AAM_FLAG,
cd.SSM_FLAG AS SSM_FLAG
FROM
(
select
case when exist (select 1 from c.clients where client_id='c-001' then 'Y' else 'N' end as ROW_EXISTS
from dual
) AS a
INNER JOIN CLIENT_DYN cd ON c.CLIENT_ID = cd.CLIENT_ID
WHERE c.CLIENT_ID = 'c-001';
Error ORA-00933: SQL command not properly ended encounters near line ) AS A when execute the query.

Answer 1

After you fix the syntax errors (EXISTS not EXIST, missing closing brace and don't use the AS keyword for table aliases), your outer query is
SELECT <some columns>
FROM (SELECT 'Y/N Value' AS row_exists FROM DUAL) a
INNER JOIN client_dyn cd
ON (c.CLIENT_ID = cd.CLIENT_ID)
There is no c table or alias in the outer-query as the sub-query is aliased to a not c and the sub-query only contains one row_exists column and not a CLIENT_ID column; so on both these points, c.CLIENT_ID is invalid.
What you probably want is something like:
If you want to check the client_id matches the current row:
SELECT CASE WHEN c.client_id IS NOT NULL THEN 'Y' ELSE 'N' END AS CLIENT_EXIST,
c.AP_Before AS AP_before,
c.AP_TIMESTAMP AS AP_TIMESTAMP,
cd.AAM_FLAG AS AAM_FLAG,
cd.SSM_FLAG AS SSM_FLAG
FROM clients c
RIGHT OUTER JOIN CLIENT_DYN cd
ON (c.CLIENT_ID = 'c-001' AND c.CLIENT_ID = cd.CLIENT_ID);
or, if you want to check if the client_id matches in the set of returned rows:
SELECT CASE
WHEN COUNT(CASE WHEN c.client_id = 'c-001' THEN 1 END) OVER () > 0
THEN 'Y'
ELSE 'N'
END AS CLIENT_EXIST,
c.AP_Before AS AP_before,
c.AP_TIMESTAMP AS AP_TIMESTAMP,
cd.AAM_FLAG AS AAM_FLAG,
cd.SSM_FLAG AS SSM_FLAG
FROM clients c
INNER JOIN CLIENT_DYN cd
ON (c.CLIENT_ID = cd.CLIENT_ID);
or, if you want to check if the client_id exists anywhere in the clients table:
SELECT CASE
WHEN EXISTS(SELECT 1 FROM clients WHERE client_id = 'c-001')
THEN 'Y'
ELSE 'N'
END AS CLIENT_EXIST,
c.AP_Before AS AP_before,
c.AP_TIMESTAMP AS AP_TIMESTAMP,
cd.AAM_FLAG AS AAM_FLAG,
cd.SSM_FLAG AS SSM_FLAG
FROM clients c
INNER JOIN CLIENT_DYN cd
ON (c.CLIENT_ID = cd.CLIENT_ID);
Depending on how you want to check if the client exists.

Answer 2

exists, not exist
missing closing subquery bracket
table aliases don't accept the as keyword (as opposed to columns)
SELECT a.ROW_EXISTS AS CLIENT_EXIST,
c.AP_Before AS AP_before,
c.AP_TIMESTAMP AS AP_TIMESTAMP,
cd.AAM_FLAG AS AAM_FLAG,
cd.SSM_FLAG AS SSM_FLAG
FROM client c
INNER JOIN CLIENT_DYN cd ON c.CLIENT_ID = cd.CLIENT_ID
CROSS JOIN (SELECT CASE
WHEN EXISTS
(SELECT 1
FROM clients c
WHERE client_id = 'c-001')
THEN
'Y'
ELSE
'N'
END AS ROW_EXISTS
FROM DUAL) a
WHERE c.CLIENT_ID = 'c-001';