Lag query pulling in incorrect data - sql

Question

I have the below table (labelled Original Table) with the following columns:
BU_Code (store code), contact_key (cutomer ID), Bu_key (store number), TXN_Mth (month of trasaction in 2021), Fragrance/Cosmetics/Personal flag (flag for type of product bought).
Original Table
I am trying to create a new table based on this which lists the previous month the customer shopped in (Pre_txn_mth) and using a CASE state to determine if they are a new customer (no previous transction before 2021), returning (shopped within 12 months) or reactivated (last shop more than 12 months ago).
However when I create the table it is listing future transactions as the previous tranaction. Below is an image from the new table in which Contact_key 1196 is pulled correctly but 1443 is not. Error in Table example
This is the code I have tried different variations of but same error:
CREATE TABLE TPS_TABLE_B AS
(
SELECT
B.*
, LAG(TXN_MTH) OVER (ORDER BY CONTACT_KEY, TXN_MTH) PRE_TXN_MTH
--, TXN_MTH - 100
, CASE
WHEN LAG(TXN_MTH) OVER (ORDER BY CONTACT_KEY, TXN_MTH) IS NULL THEN 'NEW'
WHEN TXN_MTH - 100 < (LAG(TXN_MTH) OVER (ORDER BY CONTACT_KEY, TXN_MTH)) THEN 'RETURNING'
WHEN TXN_MTH - 100 >= (LAG(TXN_MTH) OVER (ORDER BY CONTACT_KEY, TXN_MTH)) THEN 'REATIVATED'--REACTIVATED IS NO TRANSACTION IN PAST 12 MONTHS
ELSE 'OTHER'
END AS CUST_TYPE
FROM
(
SELECT
CONTACT_KEY
, BU_CODE
, BU_KEY
, TXN_MTH
, FRAGRANCE_FLAG
, COSMETICS_FLAG
, PERSONALCARE_FLAG
FROM TPS_TABLE_A
) B
)
;

Answer 1

You can create a CTE (or use this code as subquery) to get some derived values that you will use in main query to get everything you need.
WITH
c_months AS
(
Select
BU_CODE, BU_KEY, CONTACT_KEY,
TXN_MTH, FRAGRANCE_FLAG, COSMETICS_FLAG, PERSONALCARE_FLAG,
CASE WHEN SubStr(TXN_MTH - 1, -2) = '00' THEN TXN_MTH - 1 - 88 ELSE TXN_MTH - 1 END "MTH_BEFORE",
LAG(TXN_MTH) OVER(Partition By CONTACT_KEY Order By CONTACT_KEY, TXN_MTH) "MTH_PREV_TXN",
TXN_MTH - (LAG(TXN_MTH) OVER(Partition By CONTACT_KEY Order By CONTACT_KEY, TXN_MTH)) "MTHS_SINCE_PREV_TXN",
Min(TXN_MTH) OVER(Partition By CONTACT_KEY Order By CONTACT_KEY) "FIRST_MTH",
Max(TXN_MTH) OVER(Partition By CONTACT_KEY Order By CONTACT_KEY) "LAST_MTH",
Max(TXN_MTH) OVER(Partition By CONTACT_KEY Order By CONTACT_KEY) - Min(TXN_MTH) OVER(Partition By CONTACT_KEY Order By CONTACT_KEY) + 1 "TOTAL_MTHS"
From
tbl
)
/* R e s u l t :
BU_CODE BU_KEY CONTACT_KEY TXN_MTH FRAGRANCE_FLAG COSMETICS_FLAG PERSONALCARE_FLAG MTH_BEFORE MTH_PREV_TXN MTHS_SINCE_PREV_TXN FIRST_MTH LAST_MTH TOTAL_MTHS
------- ---------- ----------- ---------- -------------- -------------- ----------------- ---------- ------------ ------------------- ---------- ---------- ----------
TPS 16 1196 202108 1 0 0 202107 202108 202112 5
TPS 16 1196 202111 1 0 0 202110 202108 3 202108 202112 5
TPS 16 1196 202112 1 0 0 202111 202111 1 202108 202112 5
TPS 16 1259 202109 1 0 0 202108 202109 202109 1
TPS 16 1443 202106 1 0 0 202105 202106 202109 4
TPS 16 1443 202109 1 0 0 202108 202106 3 202106 202109 4
TPS 16 1478 202107 0 0 0 202106 202107 202107 1
TPS 16 1570 202108 1 0 0 202107 202108 202108 1
TPS 16 1637 202105 1 0 0 202104 202105 202109 5
TPS 16 1637 202106 1 0 0 202105 202105 1 202105 202109 5
TPS 16 1637 202107 1 0 0 202106 202106 1 202105 202109 5
TPS 16 1637 202109 1 0 0 202108 202107 2 202105 202109 5
TPS 16 1675 202106 1 0 0 202105 202106 202106 1
*/
Now we have more than everything we need to get you your expected result. Using the CTE's resulting dataset like below hopefuly will answer your question.
Select
BU_CODE, BU_KEY, CONTACT_KEY,
TXN_MTH,
FRAGRANCE_FLAG, COSMETICS_FLAG, PERSONALCARE_FLAG,
MTH_PREV_TXN,
CASE
WHEN MTH_PREV_TXN Is Null THEN 'NEW'
WHEN Nvl(MTHS_SINCE_PREV_TXN, 0) > 12 THEN 'REACTIVATED'
ELSE 'RETURNING'
END "CUST_TYPE"
From
c_months
With your sample data (13 rows from the question) it would reasult as:
BU_CODE
BU_KEY
CONTACT_KEY
TXN_MTH
FRAGRANCE_FLAG
COSMETICS_FLAG
PERSONALCARE_FLAG
MTH_PREV_TXN
CUST_TYPE
TPS
16
1196
202108
1
0
0
NEW
TPS
16
1196
202111
1
0
0
202108
RETURNING
TPS
16
1196
202112
1
0
0
202111
RETURNING
TPS
16
1259
202109
1
0
0
NEW
TPS
16
1443
202106
1
0
0
NEW
TPS
16
1443
202109
1
0
0
202106
RETURNING
TPS
16
1478
202107
0
0
0
NEW
TPS
16
1570
202108
1
0
0
NEW
TPS
16
1637
202105
1
0
0
NEW
TPS
16
1637
202106
1
0
0
202105
RETURNING
TPS
16
1637
202107
1
0
0
202106
RETURNING
TPS
16
1637
202109
1
0
0
202107
RETURNING
TPS
16
1675
202106
1
0
0
NEW