how to decrease by 1 a field when using $DAL

$tblitems->Value["quantity"]=$values["quantity"] - value["transaction_quantity"]; ???? this doesnt work
when i use

$tblitems->Value["quantity"]= ++$values["quantity"];
it icreases by 1 in right way , why i can't make an add or subtract?

You need custom query for this
$sql = "update Users set active=0 where id=32";

CustomQuery($sql);

I dont get it

Can you be more specific for my case?

You have several syntax errors in your coding:
Value["quantity"]-> $values["quantity"] // appeared twice

value["transaction_quantity"] -> $values["transaction_quantity"]

You have several syntax errors in your coding:
Value["quantity"]-> $values["quantity"] // appeared twice

value["transaction_quantity"] -> $values["transaction_quantity"]
i will make it more clear
Value["detailtb_quantity"]-> $values["mastertb_quantity"]

value["detailtb_transaction_quantity"] -> $values["mastertb_transaction_quantity"]
can i calculate with 2 or more fields?

if not with $DAL , how can i do it with customSQL? (PHPRUNNER/MySQL)

You need to provide more info if you want replies to be more specific. For example, the full name of your article table, how you want the quantity field in the table updated, the relation between your transaction table and article table, etc, this would be helpful for others to write specific code for you.

thanks for reply , let me explain more.
when i add a record to transaction table (using lookup to choose article from articles table)

i want While saving the transaction to update quantity of articles table
code should be like (before record added event)
global $dal;

$tblitems = $dal->Table("articles");

$tblitems->Param["articles_articlename"]= $values['transaction_article'];

$tblitems->Value["articles_quantity"]=$values["transaction_qtytotal"] - $values["transaction_quantity"]; ???? this doesnt work

$tblitems->Update();
(what should i use to Value["articles_quantity"] for quantity field to get set calculated?
PS

Phprunner/Mysql

This needs to go after record added as the values have not been added to database yet if use before add.

global $dal;

$tblarticles = $dal->Table("articles");

$tblarticles->Param["articles_articlename"]= $values['transaction_article'];

$tblarticles->Value["articles_quantity"]=($values["transaction_qtytotal"] - $values["transaction_quantity"]);

$tblarticles->Update();

yes , i have used before ADD event

I will try to move it to after ADD event
thanks for answer , i will make it and i will come back to make the thread "solved" if it works.