如何使用php减去mysql中的数量

Question

When I'm running the following query with + sign it is adding the values and showing correct result but when I replace + sign with - sign it is still showing the correct result but showing - sign before the result. Why is it displaying - sign in the result?

please help me to fix this issue thanks

ON DUPLICATE KEY UPDATE quantity='$quantity'-quantity ")

it is showing result like this

--------------
 quantity row
--------------
    -5
--------------

this is not working perfect

$result = mysql_query("INSERT INTO stock SET date='$date',company='$company',itemname='$itemname',quantity='$quantity',category='$category',signature='$signature' 
ON DUPLICATE KEY UPDATE quantity='$quantity'-quantity ")

this is working

$result = mysql_query("INSERT INTO stock SET date='$date',company='$company',itemname='$itemname',quantity='$quantity',category='$category',signature='$signature' 
ON DUPLICATE KEY UPDATE quantity='$quantity'+quantity ")

i m insert to query togetter this is my complete code but 1st one is working fine but 2nd one have problm

         // save the data to the database
          mysql_query("INSERT INTO  recivereturn SET date='$date',company='$company',itemname='$itemname',quantity='$quantity',category='$category',signature='$signature'");

$result = mysql_query("INSERT INTO stock SET date='$date',company='$company',itemname='$itemname',quantity='$quantity',category='$category',signature='$signature' 
ON DUPLICATE KEY UPDATE quantity=$quantity-quantity ")

Answer 1

You are almost certainly doing the subtraction backwards. You probably want to subtract a variable from the current value of the column, not vice versa:

ON DUPLICATE KEY UPDATE quantity = quantity - '$quantity'