Subtract two values of same column based on another column and save the result in a new row

Question

I have following table rows in SQL server.

Now, I want an end result like this (Available quantity = (Total Quantity - Rented out Quantity) WHERE YYYYMM AND BOOKNAME IS SAME)

Answer 1

To generate the 2 required rows, you can use a GROUP BY query

SELECT   t.YYYYMM, STATUS = 'AVAILABLE', t.BOOKNAME,
         SUM (CASE WHEN t.STATUS = 'TOTAL' THEN +t.QUANTITY ELSE -t.QUANTITY END) AS QUANTITY
FROM     a_table t
GROUP BY t.YYYYMM, t.BOOKNAME

You can insert that back into the table

Or

can use UNION ALL in a SELECT query when you need to display it

Answer 2

INSERT INTO table (yyyymm, status, bookname, quantity)
SELECT t1.yyyymm, 'AVAILABLE', t1.bookname, t1.quantity - t2.quantity
FROM table t1
JOIN table t2 USING (yyyymm, bookname)
WHERE t1.status = 'TOTAL'
  AND t2.status = 'RENTED OUT'

table (yyyymm, status, bookname) must be unique by proper constraint/index.

The record will be inserted only when both source records are present. If it is possible that 'RENTED OUT' is absent and its value must be assumed as zero, use LEFT JOIN ans COALESCE additionally (and move the condition by t2.status to ON clause).

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Can you please be kind enough to edit your answer to include the assumption when 'RENTED OUT' is absent?

INSERT INTO table (yyyymm, status, bookname, quantity)
SELECT t1.yyyymm, 'AVAILABLE', t1.bookname, t1.quantity - COALESCE(t2.quantity, 0)
FROM table t1
LEFT JOIN table t2 ON t1.yyyymm = t2.yyyymm
                  AND t1.bookname = t2.bookname
                  AND t2.status = 'RENTED OUT'
WHERE t1.status = 'TOTAL'

Answer 3

This query will definitely works for you please see screen shot below Thank you

create table #temp (YYYYMM varchar(10),Status varchar(20),BOOKNAME 
varchar(20),QUANTITY INT)
INSERT INTO #temp VALUES ('201912','TOTAL','PHYSICS101',260)
INSERT INTO #temp VALUES ('201912','RENTED OUT','PHYSICS101',100)
INSERT INTO #temp VALUES ('201912','TOTAL','CHEMISTRY101',250)
INSERT INTO #temp VALUES ('201912','RENTED OUT','CHEMISTRY101',50)


;WITH CTE AS (
SELECT YYYYMM,Status,BOOKNAME,QUANTITY,
CASE WHEN BOOKNAME IN('CHEMISTRY101','PHYSICS101') AND  STATUS='TOTAL' THEN QUANTITY 
END  TOTAL
,CASE WHEN BOOKNAME IN('CHEMISTRY101','PHYSICS101') AND  STATUS='RENTED OUT' THEN 
QUANTITY END RENTED_OUT
FROM #temp
) 

 SELECT YYYYMM,Status,BOOKNAME,QUANTITY FROM #temp
 UNION ALL
 SELECT YYYYMM,'AVAILABLE' Status,BOOKNAME,SUM(ISNULL(TOTAL,0)-ISNULL(RENTED_OUT,0)) 
 QUANTITY FROM CTE
 GROUP BY YYYYMM,BOOKNAME
 DROP TABLE #temp