tailieunhanh - Mastering Microsoft Visual Basic 2008 phần 9

nhưng nó làm cho hình ảnh không sử dụng được trên bất kỳ trang web khác hơn so với bản gốc. Bạn sẽ thấy làm thế nào để đặt một bán trong suốt đồ họa trên đầu trang của một hình ảnh trong chương 19, Làm việc với các mối quan hệ, chỉ số, và ràng buộc | Chapter 24 Advanced DataSet Operations As you know very well by now DataSets are miniature databases that reside in the client computer s memory. They re made up of tables related to one another they enforce relations and they re practically as close to a client-side database as you can get without installing an actual database management system at every client. However you can t execute SQL statements directly against the DataSet s data. You can t update a DataTable by issuing an UPDATE statement and can t add rows with an INSERT statement. You know how to iterate through the rows of a Data-Table and how to locate its related rows in other tables and how to select rows from a DataTable with the Select method. You can iterate through the rows of a table locate the ones that meet certain criteria and process their columns in any way you wish calculate aggregates on selected columns update rows in other tables based on the values of certain columns format text columns even create new columns and set their values. And of course you know how to edit add and delete rows and submit the DataSet to the server. You can also process the data in the DataSet at the client with LINQ. Besides LINQ to SQL which was discussed briefly in Chapter 17 Querying Collections and XML with LINQ there s another component the LINQ to DataSet component which enables you to query the DataSet s data. In this chapter I ll focus on more-traditional querying techniques. In this chapter you ll learn how to do the following Use SQL to query DataSets Add calculated columns to DataTables Compute aggregates over sets of rows Working with SQL Expressions Let s consider a DataSet that contains the Orders and Order Details tables of the Northwind database. In Chapter 22 Programming with you saw how to populate a DataSet with data from one or more database tables how to iterate through a table s rows and how to traverse the related rows. In this chapter you ll look at some more-advanced features .