PL SQL iterate loop through date range - oracle

Question

I have a date range, I am trying to take one date every week through a loop
DECLARE
start_date DATE := TO_DATE('06.01.2021', 'dd.MM.yyyy');
end_date DATE := TO_DATE('26.05.2021', 'dd.mm.yyyy');
active_date DATE;
start_number NUMBER;
end_number NUMBER;
BEGIN
start_number := TO_NUMBER(TO_CHAR(start_date, 'j'));
end_number := TO_NUMBER(TO_CHAR(end_date, 'j'));
active_date := start_date;
FOR cur_r IN start_number..end_number
LOOP
INSERT INTO test_tbl
SELECT snap_date FROM s_act
WHERE
snap_date = active_date;
active_date := TRUNC(active_date) + 7;
COMMIT;
END LOOP;
END;
When I execute this script, only one date 06.01.2021 is written to the table through all iterations.
Where am I making a mistake? How can this be fixed?

Answer 1

You do not need PL/SQL for this and can just use a recursive sub-query:
INSERT INTO test_tbl
WITH date_range ( start_date, end_date ) AS (
SELECT DATE '2021-01-06', DATE '2021-05-26' FROM DUAL
UNION ALL
SELECT start_date + INTERVAL '7' DAY,
end_date
FROM date_range
WHERE start_date + INTERVAL '7' DAY <= end_date
)
SELECT snap_date
FROM s_act s
WHERE EXISTS(
SELECT 1
FROM date_range r
WHERE r.start_date = s.snap_date
);
or a hierarchical query:
INSERT INTO test_tbl
SELECT snap_date
FROM s_act s
WHERE EXISTS(
WITH date_range ( start_date, end_date ) AS (
SELECT DATE '2021-01-06', DATE '2021-05-26' FROM DUAL
)
SELECT 1
FROM date_range r
WHERE r.start_date + ( LEVEL - 1 ) * INTERVAL '7' DAY = s.snap_date
CONNECT BY r.start_date + ( LEVEL - 1 ) * INTERVAL '7' DAY <= r.end_date
);
If you really want to use PL/SQL then you can make it much simpler and iterate by weeks rather than days (however, this will be much less efficient as you will have one INSERT per week and the associated context switch from PL/SQL to SQL compared to the SQL solution which is only a single INSERT for the entire operation and no context switches):
DECLARE
start_date DATE := DATE '2021-01-06';
end_date DATE := DATE '2021-05-26';
active_date DATE := start_date;
BEGIN
LOOP
EXIT WHEN active_date > end_date;
INSERT INTO test_tbl
SELECT snap_date FROM s_act
WHERE snap_date = active_date;
active_date := active_date + INTERVAL '7' DAY;
END LOOP;
END;
/
db<>fiddle here

Answer 2

To me, it looks as if everything is, actually, OK with code you wrote, because active_date gets its new value:
SQL> set serveroutput on;
SQL> declare
2 start_date date := to_date('06.01.2021', 'dd.MM.yyyy');
3 end_date date := to_date('26.05.2021', 'dd.mm.yyyy');
4 active_date date;
5 start_number number;
6 end_number number;
7 begin
8 start_number := to_number(to_char(start_date, 'j'));
9 end_number := to_number(to_char(end_date, 'j'));
10 active_date := start_date;
11
12 for cur_r in start_number..end_number
13 loop
14 dbms_output.put_line('Active_date = ' || to_char(active_date, 'dd.mm.yyyy'));
15 /* Commented, as I don't have your tables nor data
16 INSERT INTO test_tbl
17 SELECT snap_date
18 FROM s_act
19 WHERE snap_date = active_date;
20 */
21 active_date := trunc(active_date) + 7;
22 end loop;
23 -- move COMMIT out of the loop!
24 commit;
25 end;
26 /
Active_date = 06.01.2021
Active_date = 13.01.2021
Active_date = 20.01.2021
<snip>
Active_date = 06.09.2023
Active_date = 13.09.2023
PL/SQL procedure successfully completed.
SQL>
You said
When I execute this script, only one date 06.01.2021 is written to the table through all iterations.
This is piece of code responsible for that:
INSERT INTO test_tbl
SELECT snap_date
FROM s_act
WHERE snap_date = active_date;
I interpret it as:
s_act table contains rows only with snap_date equal to 06.01.2021, or
if it contains rows with other dates, maybe they contain a time component (hours, minutes, seconds) and where condition prevents them to be inserted. If that's so, try with
where trunc(snap_date) = active_date
and see what happens.