mySQL Queries using PHP's SESSION variables

Hello, I am stuck here. In the name of efficiency I want to lower the number of callls to the database. So I am trying to give the most stringent query possible. This is the problem: I have stored the surfers shopping cart, where each item is stored as a session variable.Now on the database I have ALL inventory items, but I want to only display the details of what the user has in his cart. eg. : I want something like this: $result = mysql_query("SELECT * from Inventory where ItemNumber is in $HTTP_SESSION_VARS"); // I need proper syntax to replace "is in" where Inventory has, ItemNumber (unique), Price, ItemName. So say the surfer has three items in the Session, then I stored the three unique ItemNumbers. Then with the above query I can get the rest of the information to represent to the user. I am looking down the avenues of a Set datastyp or maybe Enum, but I don't know if it will help.