聚合SQL Server中的求和聚合处理技巧（sqlserver求和）

Aggregating Sum Processing Techniques In SQL Server

Aggregates are functions (such as COUNT, MAX, SUM and etc.) used to perform and return a single value from multiple values from a single column, and the SUM function is used to perform the addition of data values. SQL Server SUM processing techniques aim to maximize performance for summative operations.

The simplest and most common way to implement a SUM aggregate in SQL Server is through the use of the SUM() function as part of the SELECT statement. This is a simple method with no GROUP BY clause and returns a single value.

For example, the following query returns the sum of a column called “Quantity” from the table “OrderDetail”:

“`sql

SELECT SUM(Quantity) AS TotalQuantity

FROM OrderDetail

There are other ways to produce the same result, such as using the OVER clause to create a windowing partition that provides a SUM aggregate. This partition creates a separate window for each row and each window is prepared with all the values it needs to SUM. 

For example, the following query returns the same output as the previous query using the OVER clause:
```sql
SELECT Quantity,
       SUM(Quantity) OVER() AS TotalQuantity
FROM OrderDetail

Another technique for producing the same output is a subquery within the SELECT statement. This subquery is used to contain the aggregate functions, producing an intermediate result set that is added to the main result set.

For example, the following query returns the same output as the previous two queries using a subquery within the SELECT statement:

“`sql

SELECT Quantity,

(SELECT SUM(Quantity)

FROM OrderDetail) AS TotalQuantity

FROM OrderDetail

Finally, a stored procedure can be used to execute the same query and to return the result set back to the calling program. A stored procedure is a set of SQL statements that can be used to call a set of queries and summarize the results.

For example, the following stored procedure encapsulates the query code used in the previous examples and returns the same output as the other three queries:
```sql
CREATE PROCEDURE GetTotalQuantity
AS
BEGIN
   SELECT SUM(Quantity) AS TotalQuantity
   FROM OrderDetail
END

These are some of the methods and techniques used to aggregate and perform summative operations in SQL Server. Each technique can be used to query a database and provide different performance results. Care must be taken to analyse the types of data being queried and the use cases for performance before making a decision.