mysql - MYSQL 查询，用于选择一个表的条件结果并将其显示在另一张表上

Question

如何从一个表的前一行中减去（或添加）一行（stock_details）..并在另一个表中显示结果（item_details）..？？

 1st table:stock_details
----------+----------+-----------
 stock_name | quantity | type  
------------+----------+----------
 laddu      |   200    | PURCHASE         
  Ghee      |    50    |  ISSUE        


2nd table:item_details
----------+----------
stock_name | quantity   
------------+---------

Answer 1

-- INSERT INTO item_details
--  or
-- CREATE TABLE item_details
SELECT   stock_name, SUM(CASE type
           WHEN 'PURCHASE' THEN +quantity
           WHEN 'ISSUE'    THEN -quantity
         END) AS quantity
FROM     stock_details
GROUP BY stock_name