iSeries SQL - Use Previous Record Value in JOIN criteria - sql

Question

In my example, I am trying to retrieve records from a transaction file based on the previous calendar work date. The work dates are in a file called CALNDR, the transactions in a file called TRNHST. The logic would be to use the CURRENT_DATE to set the cursor on CALNDR, then retrieve the previous work date which will be used to join to TRNHST. This would be simple enough if done in RPG but I am at a loss with SQL. Thanks, for any input you can provide.
CALNDR
TRNHST

Answer 1

This is even more simple with SQL probably.
You may run these statements as is to check.
LAG function use
WITH
CALNDR (CALDT) AS
(
VALUES
CAST ('2022-07-21' AS DATE)
, CAST ('2022-07-22' AS DATE)
, CAST ('2022-07-25' AS DATE)
)
SELECT C.*
FROM
(
SELECT CALDT, LAG (CALDT) OVER (ORDER BY CALDT) AS LAG_CALDT
FROM CALNDR
) C
-- JOIN TRNHST ...
-- You must keep this expression in WHERE >>OUTSIDE<< of the subselect
-- to get the desired result
WHERE C.CALDT =
CAST ('2022-07-25' AS DATE)
--CAST ('2022-07-24' AS DATE)
;
The problem with LAG is that you can't use it, if you have non-continuous calendar as in your case, and CURRENT_DATE for some day (as 2022-07-24 in the example) doesn't exist there.
BTW, I don't know if it's some problem namely with my IBM i v7.5 or not, but if I move the WHERE clause to the subselect, I get wrong result of the query at all - it doesn't achieve the goal desired here and doesn't return the expected result.
Aggregation function use
WITH
CALNDR (CALDT) AS
(
VALUES
CAST ('2022-07-21' AS DATE)
, CAST ('2022-07-22' AS DATE)
, CAST ('2022-07-25' AS DATE)
)
SELECT C.*
FROM
(
SELECT MAX (CALDT) AS LAG_CALDT
FROM CALNDR
WHERE CALDT <
CAST ('2022-07-25' AS DATE)
--CAST ('2022-07-24' AS DATE)
) C
-- JOIN TRNHST
;
Works disregarding of the presence of CURRENT_DATE in the calendar.

Answer 2

Honestly, if "prior working day" for a given date is something you need. You should consider adding such a column to your calendar table.
While you're at it, might as well add a "next working day".
One of the biggest benefits to a calendar table is the ability to pre-calculate such columns. Thus, greatly simplifying your statements.

Answer 3

Not real sure what LAG does as I have never used it. So, here's another solution using a cross join:
WITH calendar (workdt) AS (
VALUES (CAST('2022-01-01' AS date)),
(CAST('2022-02-01' AS date)),
(CAST('2022-03-01' AS date)),
(CAST('2022-04-01' AS date))
),
orders (orderno, orderdt) AS (
VALUES (1, CAST('2022-01-15' AS date)),
(2, CAST('2022-02-15' AS date)),
(3, CAST('2022-03-15' AS date)),
(4, CAST('2022-04-15' AS date))
)
SELECT *
FROM orders a
CROSS JOIN LATERAL (SELECT max(workdt) prevdt FROM calendar WHERE workdt < a.orderdt) b
The cross join only returns a single row for each record in the calendar file, and that row contains the largest work date less than the order date. LATERAL just lets the sub-query access columns from the outer query.

Answer 4

write an SQL function named calndr_prevWorkDate that returns the previous work date.
Then use that function in the WHERE clause of an SQL SELECT statement to select records from the TRNHST table:
select a.*
from trnhst a
where a.trandate = calndr_prevWorkDate( )
Here is the SQL function.
CREATE OR REPLACE FUNCTION calndr_prevWorkDate(
startDate date default current date )
RETURNS date
language sql
specific calndrf1
SET OPTION datfmt = *ISO, DLYPRP = *YES, DBGVIEW = *SOURCE,
USRPRF = *OWNER, DYNUSRPRF = *OWNER, COMMIT = *CHG
BEGIN
declare vCalDate date ;
declare vSqlCode decimal(5,0) default 0 ;
declare sqlCode int default 0 ;
declare sqlState char(5) default ' ' ;
declare vSqlState char(5) default ' ' ;
declare vErrText varchar(256) default '' ;
DECLARE CONTINUE HANDLER FOR SQLEXCEPTION
begin
SET vSqlCode = SQLCODE ;
SET vSqlState = SQLstate ;
get diagnostics exception 1 vErrText = message_text ;
end ;
select a.caldate
into vCalDate
from calndr a
where a.caldate < startDate
and dayofweek(a.caldate) not in ( 1, 7)
order by a.caldate desc
fetch first row only ;
return vCalDate ;
END

Answer 5

if all u need is the previous date from a table
you can use something like that
select *
from TRNHST
where Trans_Date =
(
select calendar_Date
from CALNDR
where calendar_Date < current date
order by calendar_Date desc
limit 1
)