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The user interface consists entirely of a Data Grid control named dg Main. To reset the expression value, assign an empty string or a null value to it.If you'd like to follow along, start by putting this code into the form's Load event handler: // Connect to the database Sql Connection cnn = new Sql Connection(); cnn. If you reset an expression and there is a default value specified for the column, then the default will propagate into all of the previously calculated Mike Gunderloy 05/27/2003 If you have a background in database work, you have undoubtedly run into the rules of normalization. Fill(ds Main, "Order Details"); // Relate tables ds Main. // Add a calculated column to the order details ds Main. In this particular case, the column values are calculated by using other columns in the same Data Table together with standard mathematical functions. It refers to the child Data Table related to the current Data Table.In particular, you probably internalized the rule that you should not store derived data in the database. Figure 3 shows the results: the last column in the Data Grid displays the results of the calculation for each row. So, the expression calculates the sum of the Item Total column (which is itself a calculated column) of all rows in the Order Details Data Table related to the current row in the Orders Data Table.

The BDE considers any SQL join to be read-only because inserting, updating, and deleting rows in a join is ambiguous.

For example, if your database includes an Order Details table containing columns for Price and Quantity, you should not put an Item Total column in that table. Command Text="SELECT * FROM Orders"; Sql Data Adapter da Orders = new Sql Data Adapter(); da Orders. Fill(ds Main, "Orders"); // Load order details Sql Command cmd Order Details = cnn. Finally, I can add an overall total column to the Customers Data Table, which shows the total of all orders for the customer, in this way: When a Data Table contains more than one relationship, you need to specify precisely the relation that should be used in determining the child table.

The item total can always be calculated from the price and quantity when it's needed. This is not necessary in this case because the Customers Data Table has only a single related child table.

For example, should the insert of a row into the previous join result in a new product and also a new supplier, or just a new product?

The Client Data Set/Provider architecture allows you to specify a primary update table (and advanced features actually not covered in the book) and also customize the updates' SQL, as we partially saw in Chapter 14 and we'll further explore in Chapter 16, "Multitier Data Snap Applications." ADO supports an equivalent to cached updates called batch updates, which are similar to the BDE approach.

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