Managing and manipulating Data in tables

Question

I have two Tables in my Database. The first table has columns Serial,Date,Location, Type,Condition,Color,Loaned_To, ProductNumberREF, and Department.

The next table has Product Number,Type,Brand,and Price.

With this system i am able to alter the Price of all items in the Main table without going one by one. But now i am running into a problem.

I'm creating a Total money spend Label that i want to populate with all the sums of prices of all the items in a Gridview. How do i add the Sum of all prices in a table that doesn't contain prices but has to reference to another table?

SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnString"].ConnectionString);
        conn.Open();
        SqlCommand SpentCheck = new SqlCommand("Selct Sum(Price) From Items");

this Code was my initial attempt till i realized PRICE doesn't exist in ITEMS. So how do i get the sum of all Prices in ITEMS?

UPDATE#####

I tries the first answers Suggestions this is were i am now

Answer 1

You are looking for an SQL join.

SELECT
    SUM(Price)
FROM
    Items
      INNER JOIN ProductNumber ON Items.Product_NumberREF = ProductNumber.ProductNumber