/* to sum quantity of product from two stores into one */
/* 1,2 : are ids of stores to sum their products*/ 
/* 6 : is the id of new store*/ 
INSERT INTO storedetail (
    `productid`, 
    `storeid`, 
    `productquantity`,
    `userid`,
    `storedetaildate`,
    `unittype`
)
SELECT 
    `productid`,
    6,
    sum(`productquantity`),
    `userid`,
    `storedetaildate`,
    `unittype` 
FROM 
    storedetail
WHERE 
    `storeid` in (1,2)
    group by `productid`;