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call to a member function ()free error when updating value in database

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I am trying to add a query to my script that updates a value in my database by subtracting "1".  When I run the query, I get "Fatal error: Call to a member function free() on boolean in ../path/to/my/script"

$sql = "UPDATE table_5 SET chairs = chairs - 1 WHERE chairs > 0 AND chair_model = 'model_33'";

Any idea what I'm doing wrong?


Table 5:


Chairs   |   Model Number  |

  22       |    model_33

  44       |    model_44

Edited by SF23103
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Now I have to figure out why the number is going down by two and not one!



browsers have a habit of requesting pages twice, for several different reasons. you could also have have an error in some of your client side code (submitting a form via ajax and not stopping the browser from submitting it as well) or server side code that's causing it.


in any case, you should not be modifying a count in a database column to track quantities of things. you should add a record to a database table for each 'transaction' that adds or subtracts a quantity, like what your bank or credit card company does. this record would would have columns for who (a user_id) caused the change in the quantity, the item_id, the quantity - a positive or negative value, a datatime when the records was inserted, and a status/memo column to record the type of transaction or a comment about the transaction. the initial quantity on hand would also be entered by inserting a row in the table.


to get the quantity at any point in time, you would just SUM() the quantity column value for any item_id.

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