How to put Case in Where Statement for Oracle SQL - sql

Question

For the query below, I'm trying to pull a specific date range depending on the current day of the month. If it's the 20th or less (e.g. "2/7/2020") then I want the date range for January. Otherwise, I want the date range for February. Is it possible to be done with a case statement? Or there is a better way?
SELECT
account,
start_date,
amount
FROM
table1
WHERE
CASE
WHEN (
SELECT
CAST(EXTRACT(DAY FROM sysdate) AS NUMBER)
FROM
dual
) <= 20 THEN
start_date
BETWEEN '2020-01-01' AND '2020-01-31'
ELSE start_date BETWEEN '2020-02-01' AND '2020-02-29'
END

Answer 1

You can do this by avoiding the case statement and using truncate the date - 20 to the month, e.g.:
SELECT account,
start_date,
amount
FROM table1
WHERE start_date >= TRUNC(SYSDATE - 20, 'mm')
AND start_date < add_months(TRUNC(dt - 20, 'mm'), 1);
If you really had to use a CASE expression (you can't use a CASE statement in SQL), you would need to do something like:
SELECT account,
start_date,
amount
FROM table1
WHERE start_date >= CASE WHEN to_char(SYSDATE, 'dd') <= '20' THEN add_months(TRUNC(SYSDATE, 'mm'), -1) ELSE TRUNC(SYSDATE, 'mm') END
AND start_date < CASE WHEN to_char(SYSDATE, 'dd') <= '20' THEN TRUNC(SYSDATE, 'mm') ELSE add_months(TRUNC(SYSDATE, 'mm'), 1) END;
N.B. if you're using a function, you don't need to wrap it in a select .. from dual, you can use it directly in the SQL statement.
I've also assumed that you want a dynamic range, e.g. if the day of the month is 20 or less, the range is for the previous month, otherwise the current month.
ETA: You would use the above two queries if there is an index on the start_date column, otherwise you could simply do:
SELECT account,
start_date,
amount
FROM table1
WHERE TRUNC(start_date, 'mm') = TRUNC(SYSDATE - 20, 'mm');

Answer 2

Case statements return single values. As such you should pull out the start date and you'll need two case statements.
select account, start_date, amount
from table1 where
start_date between
(case
when (select cast(extract(day from sysdate) as number) from dual) <= 20 then '2020-01-01'
else '2020-02-01'
end) and
(case
when (select cast(extract(day from sysdate) as number) from dual) <= 20 then '2020-01-31'
else '2020-02-29'
end)

Answer 3

One method subtracts 20 days and then gets the month boundary:
where start_date >= trunc(sysdate - interval '20' day, 'MON') and
start_date < trunc(sysdate - interval '20' day, 'MON') + interval '1' month
This approach is index (and partition) friendly -- an appropriate index on start_date can be used. It is also safe if start_date has time components.
Note: You can use sysdate without having to use a subquery.

Answer 4

You can use or operator with last_day function as following:
Select * from your_table
Where (
start_date <= trunc(sysdate,'mm') + 20
and start_date between trunc(sysdate,'mm') - interval '1' month and trunc(sysdate,'mm') - 1
)
Or
(
start_date > trunc(sysdate,'mm') + 20
and start_date between trunc(sysdate, 'mm') and last_day(sysdate)
)
This approach will use index on start_date, if any.
Cheers!!

Answer 5

select account, amount, start_date
from table1
where ( ( (select cast (extract (day from sysdate) as number) from dual) <= 20
and start_date between date '2020-01-01' and date '2020-01-31')
or ( (select cast (extract (day from sysdate) as number) from dual) > 20
and start_date between date '2020-02-01' and date '2020-02-29')
);

Answer 6

Using CASE expressions as BETWEEN operands:
SELECT account
, start_date
, amount
FROM table1
WHERE start_date BETWEEN CASE
WHEN extract(day from sysdate) <= 20
THEN trunc(sysdate -interval '1' month, 'month')
ELSE trunc(sysdate, 'month')
END
AND CASE
WHEN extract(day from sysdate) <= 20
THEN last_day(sysdate -interval '1' month)
ELSE last_day(sysdate)
END