Subtract Value From two tables

[datoshway]

I am building a small cart and need a way subtract a numeral from one table from another table holding the order items and quantity.  Need some help devising a query to subtract. 

 

Below is what I have, it's not right but just put it in so you can see basically what I am working with.  I'm nooby please help.  Assuming I need some kind of join statement and some help with this below?

 

$qty=$intQuantity;
mysql_query('UPDATE tblItems SET intQuantity-intQuantity WHERE intID = %d) or die(mysql_error());

 

Database structors

 

Table 1(tblItems)

intID | intQuantity

1      |  3

 

 

Table 2(tblOrderItems)

intID | intQuantity  |  intItemID

200              2                  1           

[datoshway]

Here is a little more of a developed idea of what i'm looking for then my last attempt

 

$strQuery	= sprintf(	"UPDATE tblItems SET intQuantity = intQuantity - (SELECT intQuantity FROM tblOrderItems WHERE intItemID = %d) WHERE intItemID = intID)",

 

 

[fenway]

Why the subquery -- just substract.