Get first (and last) day of month - expression simplification - sql

Question

I need to get date, which was 30 (later I will use also 90 or 18O) days before first day of current (and previous) month. I wrote this, but I think it's unnecessarily complicated, so I come here for help. It's any way how can I simplify this:
ib_encodedate(EXTRACT(YEAR from (dateadd(-30-datediff(day from cast(EXTRACT(MONTH from CURRENT_DATE)
|| '-1-'
|| EXTRACT(YEAR from CURRENT_DATE) as date) to date 'now') DAY to CURRENT_DATE))), EXTRACT(MONTH from (dateadd(-30-datediff(day from cast(EXTRACT(MONTH from CURRENT_DATE)
|| '-1-'
|| EXTRACT(YEAR from CURRENT_DATE) as date) to date 'now') DAY to CURRENT_DATE))),EXTRACT(DAY from (dateadd(-30-datediff(day from cast(EXTRACT(MONTH from CURRENT_DATE)
|| '-1-'
|| EXTRACT(YEAR from CURRENT_DATE) as date) to date 'now') DAY to CURRENT_DATE))))
In database I have dates in double, so I have to use function ib_encodedate to convert date to double and compare with date in database. Function have prototype:
ib_encodedate(INT year, INT month, INT day)
The same I need to write for last day of month.
Thanks for any help.

Answer 1

Seems like you need something like this:
SELECT
DATEADD (-EXTRACT(DAY FROM CURRENT_DATE)+1 DAY TO CURRENT_DATE) AS FIRST_DAY_OF_MONTH,
DATEADD (-30 DAY TO DATEADD (-EXTRACT(DAY FROM CURRENT_DATE)+1 DAY TO CURRENT_DATE)) AS A_MONTH_AGO,
DATEADD (-90 DAY TO DATEADD (-EXTRACT(DAY FROM CURRENT_DATE)+1 DAY TO CURRENT_DATE)) AS THREE_MONTHS_AGO,
DATEADD (-180 DAY TO DATEADD (-EXTRACT(DAY FROM CURRENT_DATE)+1 DAY TO CURRENT_DATE)) AS SIX_MONTHS_AGO
FROM
RDB$DATABASE
Using the function DATEADD from firebird you can easily accomplish this.

Answer 2

First Day of The Date:
select cast(:DATA as date) - extract(day from cast(:DATA as date)) + 1 from RDB$DATABASE
Last Day of The Date:
select cast(:DATA as date) - extract(day from cast(:DATA as date)) + 32 - extract(day from (cast(:DATA as date) - extract(day from cast(:DATA as date)) + 32)) from RDB$DATABASE

Answer 3

Using Firebird 2.5
First Day:
SELECT CAST(EXTRACT(YEAR FROM CURRENT_DATE)||'-'||EXTRACT(MONTH FROM
CURRENT_DATE)||'-01' AS DATE) FROM RDB$DATABASE
Last Day:
SELECT DATEADD (-1 DAY TO DATEADD (1 MONTH TO CAST(EXTRACT(YEAR FROM
CURRENT_DATE)||'-'||EXTRACT(MONTH FROM CURRENT_DATE)||'-01' AS DATE)))
FROM RDB$DATABASE

Answer 4

If you use firebird 1.5 you can create this function which returns the last day of the month of a specified date:
create or alter procedure LAST_DAY_OF_MONTH (
REFERENCE_DATE date)
returns (
LAST_DAY date)
as
declare variable NEXT_DAY date;
declare variable COUNTER integer;
begin
NEXT_DAY = :REFERENCE_DATE;
COUNTER = 0;
while (:COUNTER < 33) do begin
NEXT_DAY = :NEXT_DAY + 1;
if (extract(month from :NEXT_DAY) = extract(month from :REFERENCE_DATE)) then begin
LAST_DAY = :NEXT_DAY;
end
COUNTER = :COUNTER + 1;
end
suspend;
end
Then to call it just execute:
select last_day from last_day_of_month('02-23-2016')
This will return 02-29-2016

Answer 5

select dateadd (month, -1,dateadd (-extract(day from current_date)+1 day to current_date)) First_day_month_ago
from RDB$DATABASE
union all
select dateadd (-extract(day from current_date)+1 day to current_date) First_day_current_month
from RDB$DATABASE

Answer 6

Migrating from MySQL I needed the FIRST_DAY and LAST_DAY functions. Here they are:
CREATE OR ALTER FUNCTION FIRST_DAY(d TIMESTAMP)
RETURNS DATE
AS
BEGIN
RETURN CAST(d - EXTRACT(DAY FROM d) + 1 AS DATE);
END
and
CREATE OR ALTER FUNCTION LAST_DAY(d TIMESTAMP)
RETURNS DATE
AS
BEGIN
RETURN DATEADD(1 MONTH TO FIRST_DAY(d)) - 1;
END
And executing from isql:
SQL> SELECT first_day(CURRENT_TIMESTAMP), last_day(CURRENT_TIMESTAMP) FROM RDB$DATABASE rd;
FIRST_DAY LAST_DAY
=========== ===========
2020-08-01 2020-08-31

Answer 7

but creating a function like this:
create or alter function F_DAYSOFMONTH (
LMONTH integer,
LYEAR integer)
returns integer
as
declare variable LTMPDT DM_DATA;
declare variable LTMPDT2 timestamp;
begin
SELECT CAST(CAST(:lmonth AS VARCHAR(10)) || '/01/' || CAST(:lyear AS VARCHAR(10)) AS TIMESTAMP) FROM RDB$DATABASE INTO :LTMPDT;
LTMPDT2 = dateadd(1 month TO :LTMPDT);
return datediff(DAY FROM :ltmpdt TO :LTMPDT2);;
end^
SET TERM ; ^

Answer 8

CREATE PROCEDURE FIRST_LAST_DAYOFMONTH(COMPARE_DATE TIMESTAMP)
RETURNS (FIRST_DAYOFOMONTH TIMESTAMP,LAST_DAYOFMONTH TIMESTAM)
AS
BEGIN
FIRST_DAYOFMONTH=COMPARE_DATE-(EXTRACT(DAY FROM :COMPARE_DATE)-1);
LAST_DAYOFMONTH=DATEADD(1 MONT TO :COMPARE_DATE)-1;
SUSPEND;
END^