Hierarchical query using oracle SQL

Question

We have following table to query data from,

EMPLOYEE_ABSENCE_TAB

emp_id	absence_id	from_date	to_date	absence_continuation
100	1	01/01/2022	03/01/2022
100	2	01/02/2022	05/02/2022	1
200	3	01/01/2022	07/01/2022
200	4	10/01/2022	14/01/2022	3
200	5	16/01/2022	20/01/2022	4
300	6	01/01/2022	14/01/2022

We need to connect data hierarchically based on the following logic. Output should be emp_id, absence_days, from_date and to_date.

• absence_days : number of absence days = sum(to_date - from_date)
• from_date : first from_date where absence_continuation is NULL
• to_date : last to_date connect by prior absence_id = absence_continuation

---

Expected Outcome

emp_id	absence_days	from_date	to_date
100	8	01/01/2022	05/02/2022
200	17	01/01/2022	20/01/2022
300	14	01/01/2022	14/01/2022

I tried hierarchical but could able to come up with working solution. How can this be achieved using hierarchical query in Oracle SQL?

Answer 1

You don't need to consider the hierarchy in the data (assuming that there are no overlaps); you just need to aggregate:

SELECT emp_id,
       SUM(to_date - from_date + 1) AS absence_days,
       MIN(from_date) AS from_date,
       MAX(to_date) AS to_date
FROM   EMPLOYEE_ABSENCE_TAB
GROUP BY emp_id

Which, for the sample data:

CREATE TABLE EMPLOYEE_ABSENCE_TAB
(emp_id, absence_id, from_date, to_date, absence_continuation) AS
SELECT 100, 1, DATE '2022-01-01', DATE '2022-01-03', NULL FROM DUAL UNION ALL
SELECT 100, 2, DATE '2022-02-01', DATE '2022-02-05', 1    FROM DUAL UNION ALL
SELECT 200, 3, DATE '2022-01-01', DATE '2022-01-07', NULL FROM DUAL UNION ALL
SELECT 200, 4, DATE '2022-01-10', DATE '2022-01-14', 3    FROM DUAL UNION ALL
SELECT 200, 5, DATE '2022-01-16', DATE '2022-01-20', 4    FROM DUAL UNION ALL
SELECT 300, 6, DATE '2022-01-01', DATE '2022-01-14', NULL FROM DUAL;

Outputs:

EMP_ID	ABSENCE_DAYS	FROM_DATE	TO_DATE
100	8	2022-01-01 00:00:00	2022-02-05 00:00:00
200	17	2022-01-01 00:00:00	2022-01-20 00:00:00
300	14	2022-01-01 00:00:00	2022-01-14 00:00:00

---

If you did want to consider the hierarchy and get the duration of each absence (and not the total absences per employee) then you can use:

SELECT emp_id,
       absence_id,
       SUM(to_date - from_date + 1) AS absence_days,
       MIN(from_date) AS from_date,
       MAX(to_date) AS to_date
FROM   (
  SELECT emp_id,
         CONNECT_BY_ROOT absence_id AS absence_id,
         to_date - from_date + 1 AS absence_days,
         from_date,
         to_date
  FROM   EMPLOYEE_ABSENCE_TAB
  START WITH absence_continuation IS NULL
  CONNECT BY PRIOR absence_id = absence_continuation
)
GROUP BY
       emp_id,
       absence_id

Which outputs the same (with an additional absence_id column for the id of the first part of the absence) for your sample data as you only have a single absence for each employee; if you had more absences then it would aggregate each absence separately.

db<>fiddle here