Như bạn thấy, các truy vấn đầu tiên trả về các ấn phẩm của tác giả thứ tự tăng dần thứ tự chữ cái (mặc định), và trả về thứ hai họ theo tên thứ tự giảm dần. Nếu bạn muốn sắp xếp tất cả các hàng của tác giả và sau đó giảm dần theo năm xuất bản (để thấy gần đây nhất đầu tiên), bạn sẽ phát hành các truy vấn sau đây: | Figure 8-16. Sorting the results of requests ORDER BY ORDER BY sorts returned results by one or more columns in ascending or descending order. Example 8-27 shows two such queries the results of which can be seen in Figure 8-16. Example 8-27. Using ORDER BY SELECT author title FROM classics ORDER BY author SELECT author title FROM classics ORDER BY title DESC As you can see the first query returns the publications by author in ascending alphabetical order the default and the second returns them by title in descending order. If you wanted to sort all the rows by author and then by descending year of publication to view the most recent first you would issue the following query SELECT author title year FROM classics ORDER BY author year DESC This shows that each ascending and descending qualifier applies to a single column. The DESC keyword applies only to the preceding column year. Because you allow author to use the default sort order it is sorted in ascending order. You could also have explicitly specified ascending order for that column with the same results SELECT author title year FROM classics ORDER BY author ASC year DESC GROUP BY In a similar fashion to ORDER BY you can group results returned from queries using GROUP BY which is good for retrieving information about a group of data. For example if you want to know how many publications there are of each category in the classics table you can issue the following query Indexes 191 SELECT category COUNT author FROM classics GROUP BY category which returns the following output ------------------ ------------- category COUNT author ------------------ ------------- Classic Fiction 3 Non-Fiction 1 Play 1 ------------------ ------------- 3 rows in set sec Joining Tables Together It is quite normal to maintain multiple tables within a database each holding a different type of information. For example consider the case of a customers table that needs to be able to be cross-referenced with publications purchased .
