exclude part of the select not to consider date where clause - oracle

Question

i have a select(water readings, previous water reading, other columns) , a "where clause" that is based on date water reading date. however for previous water reading it must not consider the where clause. I want to get previous meter reading regardless where clause date range.
looked at union problem is that i have to use the same clause,
SELECT
WATERREADINGS.name,
WATERREADINGS.date,
LAG( WATERREADINGS.meter_reading,1,NULL) OVER(
PARTITION BY WATERREADINGS.meter_id,WATERREADINGS.register_id
ORDER BY WATERREADINGS.meter_id DESC,WATERREADINGS.register_id
DESC,WATERREADINGS.readingdate ASC,WATERREADINGS.created ASC
) AS prev_water_reading,
FROM WATERREADINGS
WHERE waterreadings.waterreadingdate BETWEEN '24-JUN-19' AND
'24-AUG-19' and isactive = 'Y'
The prev_water_reading value must not be restricted by the date BETWEEN '24-JUN-19' AND '24-AUG-19' predicate but the rest of the sql should be.

Answer 1

You can do this by first finding the previous meter readings for all rows and then filtering those results on the date, e.g.:
WITH meter_readings AS (SELECT waterreadings.name,
waterreadings.date dt,
lag(waterreadings.meter_reading, 1, NULL) OVER (PARTITION BY waterreadings.meter_id, waterreadings.register_id
ORDER BY waterreadings.readingdate ASC, waterreadings.created ASC)
AS prev_water_reading,
FROM waterreadings
WHERE isactive = 'Y')
-- the meter_readings subquery above gets all rows and finds their previous meter reading.
-- the main query below then applies the date restriction to the rows from the meter_readings subquery.
SELECT name,
date,
prev_water_reading,
FROM meter_readings
WHERE dt BETWEEN to_date('24/06/2019', 'dd/mm/yyyy') AND to_date('24/08/2019', 'dd/mm/yyyy');

Answer 2

Perform the LAG in an inner query that is not filtered by dates and then filter by the dates in the outer query:
SELECT name,
"date",
prev_water_reading
FROM (
SELECT name,
"date",
LAG( meter_reading,1,NULL) OVER(
PARTITION BY meter_id, register_id
ORDER BY meter_id DESC, register_id DESC, readingdate ASC, created ASC
) AS prev_water_reading,
waterreadingdate --
FROM WATERREADINGS
WHERE isactive = 'Y'
)
WHERE waterreadingdate BETWEEN DATE '2019-06-24' AND DATE '2019-08-24'
You should also not use strings for dates (that require an implicit cast using the NLS_DATE_FORMAT session parameter, which can be changed by any user in their own session) and use date literals DATE '2019-06-24' or an explicit cast TO_DATE( '24-JUN-19', 'DD-MON-RR' ).
You also do not need to reference the table name for every column when there is only a single table as this clutters up your code and makes it difficult to read and DATE is a keyword so you either need to wrap it in double quotes to use it as a column name (which makes the column name case sensitive) or should use a different name for your column.

Answer 3

I've added a subquery with previous result without filter and then joined it with the main table with filters:
SELECT
WATERREADINGS.name,
WATERREADINGS.date,
w_lag.prev_water_reading
FROM
WATERREADINGS,
(SELECT name, date, LAG( WATERREADINGS.meter_reading,1,NULL) OVER(
PARTITION BY WATERREADINGS.meter_id,WATERREADINGS.register_id
ORDER BY WATERREADINGS.meter_id DESC,WATERREADINGS.register_id
DESC,WATERREADINGS.readingdate ASC,WATERREADINGS.created ASC
) AS prev_water_reading
FROM WATERREADINGS) w_lag
WHERE waterreadings.waterreadingsdate BETWEEN '24-JUN-19' AND '24-AUG-19' and isactive = 'Y'
and WATERREADINGS.name = w_lag.name
and WATERREADINGS.date = w_lag.date