Subtract one value from another value on different rows - sql

Question

I have data that is imported daily into a table called TEMP. The value in the Amount field represents a running total which resets to zero on the first day of each month. I need to subtract the previous day's value from the current day's value before inserting a record for the adjusted value into the ACT_M table. The first day of the month is accounted for so I just need to figure out how to make the adjustment when the current day is not the first of the month. Below is an example the TEMP table and the desired results of the ACT_M table using Jan 20 as the current day and Jan 19 as the previous day. The Dept and Type columns must be treated as key fields as some Dept's can have multiple Type's and there will always be two records for each Dept and Type combination. There is also a possibility of negative values. Some Dept's, like 10000087, are used as transitionary Dept's and the amounts can later be adjusted and credited to a regular Dept resulting in negative values.
TABLE "TEMP"
DATE
DEPT
TYPE
AMOUNT
20230120
10000064
AC
525
20230119
10000064
AC
498
20230120
10000064
A
696
20230119
10000064
A
667
20230120
10000066
A
731
20230119
10000066
A
707
20230120
10000067
O
182
20230119
10000067
O
175
20230120
10000068
A
641
20230119
10000068
A
611
20230120
10000087
A
-5
20230119
10000087
A
-4
TABLE "ACT_M"
DATE
DEPT
TYPE
AMOUNT
20230120
10000064
AC
27
20230120
10000064
A
29
20230120
10000066
A
24
20230120
10000067
O
7
20230120
10000068
A
30
20230120
10000087
A
-1
I tried modifying code snippets I found online with no luck. I'm totally out of my element here and don't know if some type of join would make more sense. I last tried using examples I found for the LAG function but can't resolve the errors. Says I the function must have an OVER clause. Let me know if additional information is needed.
SELECT Date, Dept, Type, Amount,
LAG(Amount) AS PrvDay,
Amount - LAG(Amount)
OVER (ORDER BY Date) AS Diff_PrvDay
FROM {temp}

Answer 1

WITH CTE AS (
SELECT *,
ROW_NUMBER() OVER (PARTITION BY Dept, Type ORDER BY Date) AS RowNum
FROM TEMP
)
SELECT
T1.Date,
T1.Dept,
T1.Type,
T1.Amount - T2.Amount AS Amount
FROM
CTE T1
LEFT JOIN CTE T2 ON T1.Dept = T2.Dept
AND T1.Type = T2.Type
AND T1.RowNum = T2.RowNum + 1
WHERE
T1.RowNum > 1;
TEST
http://sqlfiddle.com/#!18/881df/1