CASE WHEN to choose which list of values to bring back ORACLE SQL - sql

Question

I am trying to use a case when to enforce some data access security that's not standard.
The basics of what I am trying to do keep causing the "more than one row" error.
I want to use 2 different subqueries with the result of the case when determining which list will be returned.
Example logic
SELECT CASE
WHEN condition 1 is true THEN (SUBQUERY 1)
WHEN condition 2 is true THEN (SUBQUERY 2)
END AS ACCESS
FROM DUAL
Both subqueries on there own work fine and fetch back correct values, put them into the brackets above and it fails.
SUBQUERY 1
SELECT DISTINCT FND.PK1_START_VALUE
FROM FND_GRANTS FG
JOIN FND_OBJECTS_VL FO
ON FO.OBJECT_ID = FG.OBJECT_ID
JOIN FND_FORM_FUNCTIONS_VL FFF
ON FO.OBJECT_ID = FFF.OBJECT_ID
JOIN FND_MENU_ENTRIES FME
ON FME.MENU_ID = FG.MENU_ID AND FFF.FUNCTION_ID = FME.FUNCTION_ID
LEFT JOIN FND_OBJECT_INSTANCE_SETS_VL FOIS
ON FG.INSTANCE_SET_ID = FOIS.INSTANCE_SET_ID
JOIN FND_SESSION_ROLES FSR
ON FG.ROLE_NAME = FSR.ROLE_NAME
JOIN FND_SESSIONS FS
ON FS.SESSION_ID = FSR.SESSION_ID
LEFT JOIN FND_FLEX_VALUES_VL FLV
ON FLV.DESCRIPTION = SUBSTR(FG.NAME, 1, CASE WHEN INSTR(FG.NAME,'Area')< 1 THEN INSTR(FG.NAME,'Cost')-2)
ELSE (INSTR(FG.NAME,'Area')-2)
END)
JOIN FND_TREE_NODE FND
ON FLV.FLEX_VALUE = FND.PARENT_PK1_VALUE
WHERE FG.ROLE_NAME IN (:P_ROLE)----:P_ROLE is a list of VALUES fetching back the users system access.
AND FND.TREE_STRUCTURE_CODE = 'GL_ACCT_FLEX'
AND FND.TREE_CODE ='Cost Centre-T'
AND FND.DEPTH ='4'
ORDER BY FND.PK1_START_VALUE ASC)
SUBQUERY 2----idea here is the user has none of the roles in P_ROLE and therefore has all access to all sections
SELECT DISTINCT FND.PK1_START_VALUE
FROM FND_TREE_NODE FND
WHERE FND.TREE_STRUCTURE_CODE IN('GL_ACCT_FLEX')
AND FND.TREE_CODE IN('Cost Centre-T')
AND FND.DEPTH IN('4')
NOTE some sensitive data has been redacted from the query and replaced

Answer 1

One (simple) option is to UNION them:
select columns
from subquery_1
where condition = 1
union all
select columns
from subquery_2
where condition = 2

Answer 2

Hmmm . . . I think you want union all:
SELECT q1.*
FROM (SUBQUERY 1) q1
WHERE condition1
UNION ALL
SELECT q2.*
FROM (SUBQUERY 2) q1
WHERE (NOT condition 1) AND -- may not be necessary
(condition 2) is true ;
Note: This assumes that the column(s) returned by each subquery are compatible -- in both number and types. The name in the result set is taken from the first subquery.

Answer 3

You can do something like the below using CASE
WITH sub1
AS (SELECT ROWNUM rw,
data
FROM sub_query1),
sub2
AS (SELECT ROWNUM rw,
data
FROM sub_query2)
SELECT CASE
WHEN condition1 = 1 THEN a1.data
WHEN condition1 = 2 THEN b1.data
ELSE NULL
END case1
FROM sub1 a1
full outer join sub2 b1
ON a1.rw = b1.rw