How to nest multiple case when expressions and add a condition - sql

Question

I am trying to divide customers (contact_key) column who shopped in 2021 (A.TXN_MTH) into new and 'returning' with returning meaning that they had not shopped in the last 12 months (YYYYMM in X.Fiscal_mth_idnt column).
I am using CASE WHEN A.TXN_MTH = MIN(X.FISCAL_MTH_IDNT) THEN 'NEW' which is correct. The next case when should be when the max month before X.TXN_MTH is 12 or more months previous. I have added the 12 months part in the Where statement. Should I be nesting 3 CASE WHEN'S instead of WHERE?
SELECT
T.CONTACT_KEY
, A.TXN_MTH
, CASE WHEN A.TXN_MTH = MIN(X.FISCAL_MTH_IDNT) THEN 'NEW'
WHEN (MAX(CASE WHEN X.FISCAL_MTH_IDNT < A.TXN_MTH THEN X.FISCAL_MTH_IDNT ELSE NULL END)) THEN 'RETURNING'
END AS CUST_TYPE
FROM B_TRANSACTION T
INNER JOIN B_TIME X
ON T.TRANSACTION_DT_KEY = X.DATE_KEY
INNER JOIN A
ON A.CONTACT_KEY = T.CONTACT_KEY AND A.BU_KEY = T.BU_KEY
WHERE (MAX(CASE WHEN X.FISCAL_MTH_IDNT < A.TXN_MTH THEN X.FISCAL_MTH_IDNT ELSE NULL END)) < A.TXN_MTH - (date_format(add_months(concat_ws('-',substr(yearmonth,1,4),substr(yearmonth,5,2),'01'),-12),'yyyyMM')
GROUP BY
T.CONTACT_KEY
, TXN_MTH;

Answer 1

You have not described your tables, so assuming fiscal_mth_idnt is a DATE column then you can use the LAG analytic function to find the previous row's value:
SELECT contact_key,
txn_mth,
CASE
WHEN prev_fiscal_mth_idnt IS NULL
THEN 'NEW'
WHEN ADD_MONTHS(prev_fiscal_mth_idnt, 12) < fiscal_mth_idnt
THEN 'RETURNING'
ELSE 'CURRENT'
END AS cust_type
FROM (
SELECT T.CONTACT_KEY,
A.TXN_MTH,
yearmonth,
X.FISCAL_MTH_IDNT,
LAG(X.FISCAL_MTH_IDNT) OVER (
PARTITION BY T.CONTACT_KEY
ORDER BY X.FISCAL_MTH_IDNT
) AS prev_fiscal_mth_idnt
FROM B_TRANSACTION T
INNER JOIN B_TIME X
ON T.TRANSACTION_DT_KEY = X.DATE_KEY
INNER JOIN A
ON A.CONTACT_KEY = T.CONTACT_KEY AND A.BU_KEY = T.BU_KEY
)
WHERE yearmonth LIKE '2021%';