五种提高SQL性能的方法(一)

译文
Five Ways to Rev up Your SQL Performance
 Sometimes all it takes is a little tweak here or there to make your application run much faster. Ah, but the key is figuring out how to tweak it! Sooner or later you'll face a situation where a SQL query in your application isn't responding the way you intended. Either it doesn't return the data you want or it takes entirely too long to be reasonable. If it slows down a report or your enterprise application, users won't be pleased if they have to wait inordinate amounts of time. And just like your parents didn't want to hear why you were coming in past curfew, users don't want to hear why your query is taking so long. ("Sorry, Mom, I used too many LEFT JOINs.") Users want applications to respond quickly and their reports to return analytical data in a flash. I myself get impatient when I surf the Web and a page takes more than ten seconds to load (OK, more like five seconds).
 To resolve these issues, it is important to get to the root of the problem. So where do you start? The root cause is usually in the database design and the queries that access it. In this month's column I'll demonstrate four techniques that can be used to either improve your SQL Server™-based application's performance or improve its scalability. I'll examine the use of LEFT JOINs, CROSS JOINs, and retrieving an IDENTITY value. Keep in mind that there is no magic solution. Tuning your database and its queries takes time, analysis, and a lot of testing. While the techniques here are proven, some may work better than others in your application.
 Returning an IDENTITY From an INSERT
 I figured I would start with something I get a lot of questions about: how to retrieve an IDENTITY value after performing a SQL INSERT. Often, the problem is not how to write the query to retrieve the value, but rather where and when to do it. In SQL Server, the statement to retrieve the IDENTITY value created by the most recent SQL statement run on the active database connection is as follows:
 SELECT @@IDENTITY
 While this SQL is far from daunting, it is important to keep in mind that if the most recent SQL statement was not an INSERT or you run this SQL against a different connection than the INSERT SQL, you will not get back the value you expect. You must run this code to retrieve the IDENTITY immediately following the INSERT SQL and on the same connection, like this:
 INSERT INTO Products (ProductName) VALUES ('Chalk')
 SELECT @@IDENTITY
 Running these queries on a single connection against the Northwind database will return to you the IDENTITY value for the new product called Chalk. So in your Visual Basic® application using ADO, you could run the following statement:
 Set oRs = oCn.Execute("SET NOCOUNT ON;INSERT INTO Products _
 (ProductName) VALUES ('Chalk');SELECT @@IDENTITY")
 lProductID = oRs(0)
 This code tells SQL Server not to return a row count for the query, then executes the INSERT statement and returns the IDENTITY value just created for the new row. The SET NOCOUNT ON statement means the Recordset that is returned has one row and one column that contains the new IDENTITY value. Without this statement, an empty Recordset is returned (because the INSERT statement returns no data) and then a second Recordset is returned, which contains the IDENTITY value. This can be confusing, especially since you never intended the INSERT to return a Recordset. This situation occurs because SQL Server sees the row count (that is, one row affecte

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