Oracle SQL CTE (Common Table Expression) where no data - sql

Question

I'm building a very complex SQL query in Oracle using multiple CTEs, where latter expressions rely on values from former ones. However I find that the whole execution halts if one of the prior CTEs contains no data. For example:
WITH CTE1 AS
(
SELECT
PEOPLE.ID AS PID,
APPLICATIONS.DATE AS APPDATE
FROM
PEOPLE,
APPLICATIONS
WHERE
APPLICATIONS.PERSON_ID = PEOPLE.ID
AND APPLICATIONS.DATE BETWEEN TO_DATE('2015-01-01','YYYY-MM-DD') AND TO_DATE('2015-01-31','YYYY-MM-DD')
),
CTE2 AS
(
SELECT
APPLICATIONS.PERSON_ID AS PID
MIN(APPLICATIONS.DATE) AS EARLIEST_APPDATE
FROM
CTE1,
APPLICATIONS
WHERE
APPLICATIONS.PERSON_ID = CTE1.PID
AND APPLICATIONS.DATE < ADD_MONTHS(CTE1.APPDATE, -18)
GROUP BY APPLICATIONS.PERSON_ID
),
MAIN_QUERY AS
(
SELECT
CTE1.PID AS PID
FROM
CTE1, CTE2
WHERE
-- Note that the PIDs should either match, or should not exist in CTE2
CTE1.PID = CTE2.PID OR (NOT EXISTS (SELECT PID FROM CTE2 WHERE CTE1.PID = CTE2.PID))
)
SELECT
MAIN_QUERY.PID
FROM MAIN_QUERY
Of course, I realise that the above example is completely pointless in itself, however I have just simplified this to illustrate the problem. CTE2 returns the earliest date of any application made by the same Person ID where the application is dated more than 18 months prior to the application date of CTE1. However... what if there are no such applications? CTE2 is capable of returning zero rows.
You will notice that CTE2 is not, in itself, referenced in the final query. An empty CTE2 is dealt with in MAIN_QUERY. So with regards the final query, it should not matter whether CTE2 actually returns any lines or not.
However the application I'm using (Business Objects) throws up an error that the query has "no data to fetch", when CTE2 has no lines.
I want to find a way around this, to enable my query to execute even if CTE2 returns null. Thanks.

Answer 1

CTE2 returns the earliest date of any application made by the same Person ID where the application is dated more than 18 months prior to the application date of CTE1. However... what if there are no such applications? CTE2 is capable of returning zero rows.
You can replace your sub-query factoring (WITH ... AS ( ... )) clauses with a simple analytical function:
Oracle Setup:
CREATE TABLE PEOPLE ( id ) AS
SELECT LEVEL FROM DUAL CONNECT BY LEVEL <= 3;
CREATE TABLE APPLICATIONS ( id, person_id, "DATE" ) AS
SELECT 1, 1, DATE '2015-01-01' FROM DUAL UNION ALL -- First row to return
SELECT 2, 1, DATE '2014-01-01' FROM DUAL UNION ALL -- Within 18 months
SELECT 3, 1, DATE '2013-01-01' FROM DUAL UNION ALL -- Before 18 months
SELECT 4, 1, DATE '2012-01-01' FROM DUAL UNION ALL -- Before 18 months and min
SELECT 5, 2, DATE '2015-01-02' FROM DUAL UNION ALL -- Second row to return
SELECT 6, 2, DATE '2014-01-02' FROM DUAL UNION ALL -- Within 18 months
SELECT 7, 3, DATE '2015-01-03' FROM DUAL UNION ALL -- Third row to return
SELECT 8, 3, DATE '2013-07-03' FROM DUAL; -- Exactly 18 months earlier
Query:
SELECT PID,
APPDATE,
CASE EARLIEST_APPDATE
WHEN APPDATE - INTERVAL '18' MONTH
THEN NULL
ELSE EARLIEST_APPDATE
END AS EARLIEST_APPDATE -- Included for the edge case where
-- EARLIEST_APPDATE is exactly 18 months
-- earlier as the RANGE BETWEEN is
-- inclusive.
FROM (
SELECT p.ID AS PID,
a."DATE" AS APPDATE,
MIN( a."DATE" ) OVER ( PARTITION BY p.ID
ORDER BY a."DATE"
RANGE BETWEEN UNBOUNDED PRECEDING
AND INTERVAL '18' MONTH PRECEDING )
AS EARLIEST_APPDATE
FROM PEOPLE p
INNER JOIN APPLICATIONS a
ON ( a.PERSON_ID = p.ID )
)
WHERE APPDATE BETWEEN DATE '2015-01-01' AND DATE '2015-01-31'
Output:
PID APPDATE EARLIEST_APPDATE
---------- ------------------- -------------------
1 2015-01-01 00:00:00 2012-01-01 00:00:00
2 2015-01-02 00:00:00
3 2015-01-03 00:00:00