Teradata SQL: Determine how many accounts had status change in given month - sql

Question

Ok, so I have a table that looks something like this:
Acct_id Eff_dt Expr_dt Prod_cd Open_dt
-------------------------------------------------------
111 2012-05-01 2013-06-01 A 2012-05-01
111 2013-06-02 2014-03-08 A 2012-05-01
111 2014-03-09 9999-12-31 B 2012-05-01
222 2015-07-15 2015-11-11 A 2015-07-15
222 2015-11-12 2016-08-08 B 2015-07-15
222 2016-08-09 9999-12-31 A 2015-07-15
333 2016-01-01 2016-04-15 B 2016-01-01
333 2016-04-16 2016-08-08 B 2016-01-01
333 2016-08-09 9999-12-31 A 2016-01-01
444 2017-02-03 2017-05-15 A 2017-02-03
444 2017-05-16 2017-12-02 A 2017-02-03
444 2017-12-03 9999-12-31 B 2017-02-03
555 2017-12-12 9999-12-31 B 2017-12-12
There are many more columns that I'm not including as they're otherwise not relevant.
What I'm trying to determine is how many accounts had a change in Prod_cd in a given month, but then only in one direction (so from A > B in this example). Sometimes however an account was first opened as B, and then later changed to A. Or it was opened as A, changed to B, and moved back to A. I only want to know the current set of accounts where in a given month the Prod_cd changed from A to B.
Eff_dt is the date when a change was made to an account (could be any change, such as address change, name change, or what I'm looking for, product code change).
Expr_dt is the expiration date of that row, essentially the last day before a new change was made. When the date of that row is 9999-12-31, that's the most current row.
Open_dt is the date the account was created.
I created a query at first that was something like this:
select
count(distinct acct_id)
from table
where prod_cd = 'B'
and expr_dt = '9999-12-31'
and eff_dt between '2017-12-01' and '2017-12-31'
and open_dt < '2017-12-01'
But it's giving me results that don't look right. I want to specifically track the # of conversions that happened, but the count of accounts I'm getting seems way too high.
There is probably a way to create a more reliable query using window functions, but given that the Prod_cd changes can happen in multiple directions, I'm not sure how to write that query. Any help would be appreciated!

Answer 1

If you are specifically looking for the switch A --> B, then the simplest method is to use lag(). But, Teradata requires a slightly different formulation:
select count(distinct acct_id)
from (select t.*,
max(prod_cd) over (partition by acct_id order by effdt rows between 1 preceding and 1 preceding) as prev_prod_cd
from t
) t
where prod_cd = 'B' and prev_prod_cd = 'A' and
expr_dt = '9999-12-31' and
eff_dt between '2017-12-01' and '2017-12-31' and
open_dt < '2017-12-01';
I am guessing that the date conditions go in the outer query -- meaning that they lag() does not use them.

Answer 2

Similar to Gordon's answer, but using a supported window function (instead of LAG) and using Teradata's QUALIFY clause to do the lag-gy lookup:
SELECT DISTINCT acct_id
FROM mytable
QUALIFY
MAX(prod_cd) OVER (PARTITION BY acct_id ORDER BY eff_dt ASC ROWS BETWEEN 1 PRECEDING AND 1 PRECEDING) = 'A'
AND prod_cd = 'B'
AND expr_dt = '9999-12-31'
AND eff_dt between DATE '2013-01-01' and DATE '2017-12-31'
AND open_dt < DATE '2017-12-01'