tailieunhanh - Hướng dẫn học Microsoft SQL Server 2008 part 34

Bởi vì bây giờ đã trở về SQL thông tin từ một tập hợp, thay vì xây dựng một hồ sơ đặt hàng, ngay sau khi một truy vấn bao gồm một chức năng tổng hợp, mỗi cột (trong danh sách cột, trong biểu thức, hoặc trong TỰ DO) phải tham gia một chức năng tổng hợp. | Part II Manipulating Data With Select Because SQL is now returning information from a set rather than building a record set of rows as soon as a query includes an aggregate function every column in the column list in the expression or in the ORDER BY must participate in an aggregate function. This makes sense because if a query returned the total number of order sales then it could not return a single order number on the summary row. Because aggregate functions are expressions the result will have a null column name. Therefore use an alias to name the column in the results. To demonstrate the mathematical aggregate functions the following query produces a SUM AVG MIN and MAX of the amount column. SQL Server warns in the result that null values are ignored by aggregate functions which are examined in more detail soon SELECT SUM Amount AS Sum AVG Amount AS Avg MIN Amount AS Min MAX Amount AS Max FROM RawData Result Sum Avg Min Max 946 47 11 91 Warning Null value is eliminated by an aggregate or other SET operation. There s actually more to the COUNT function than appears at first glance. The next query exercises four variations of the COUNT aggregate function SELECT COUNT AS CountStar COUNT RawDatalD AS CountPK COUNT Amount AS CountAmount COUNT DISTINCT Region AS Regions FROM RawData Result CountStar CountPK CountAmount Regions 24 24 20 4 Warning Null value is eliminated by an aggregate or other SET operation. To examine this query in detail the first column COUNT counts every row regardless of any values in the row. COUNT RawDataID counts all the rows with a non-null value in the primary key. Because primary keys by definition can t have any nulls this column also counts every row. These two methods of counting rows have the same query execution plan same performance and same result. The third column COUNT Amount demonstrates why every aggregate query includes a warning. It counts the number of rows with an actual value in the Amount column and it ignores any rows .

• Cơ sở dữ liệu
• Giáo trình SQL Server 2008
• Microsoft SQL Server 2008
• quản trị dữ liệu
• cài đặt SQL Server 2008
• công cụ của SQL Server 2008
• Giáo trình Microsoft SQL Server 2008
• Quản trị cơ sở dữ liệu
• Cài đặt Microsoft SQL Server 2008
• Cấu trúc trình đơn của Microsoft SQL Server 2008
• Cơ sở dữ liệu SQL Server 2008
• Dịch vụ Report trong SQL Server 2008
• SQL Server Business Intelligence Development Studio
• Bảo mật cơ sở dữ liệu SQL Server 2008
• SQL Server 2008
• Cơ bản SQL Server 2008
• Kiến trúc SQL Server 2008
• Phục hồi cơ sở dữ liệu