Tài liệu MASTERING SQL SERVER 2000- P7 doc

CHAPTER 8 • ADVANCED TRANSACT-SQL 270 Committing a nested transaction does not write the changes from that transaction permanently to the database; it merely makes them available to the outer transaction. Suppose you have the following SQL batch: BEGIN TRANSACTION UPDATE titles SET price = 20.00 WHERE title_id = ‘TC7777’ BEGIN TRANSACTION UPDATE titles SET type = ‘potboiler’ WHERE title_id = ‘TC7777’ COMMIT TRANSACTION ROLLBACK TRANSACTION In this case, the COMMIT TRANSACTION statement tells SQL Server that you’re finished with the second transaction that you started. However, the ROLLBACK TRANSACTION then rolls back all the work since the first BEGIN TRANSACTION, including the inner nested transaction. Although transaction names appear to offer increased readability for your code, they interact poorly with nested transactions. In fact, you can refer to a transaction by name only if it’s the outermost transaction in a batch. Our recommendation is to avoid naming transactions if you plan to ever nest transactions. COMMIT TRANSACTION The syntax of COMMIT TRANSACTION is very similar to that of BEGIN TRANSAC- TION. There’s also an alternative statement with the same purpose: COMMIT TRANS[ACTION] [transaction_name | @name_variable] COMMIT [WORK] When you issue a COMMIT TRANSACTION statement, the most recent transaction you started is marked as ready to commit. When you commit the outermost in a series of nested transactions, the changes are written back to the database. Of course, if there’s only one transaction open, the changes are written immediately. It’s your responsibility to make sure you’ve made all the changes you want before issuing a COMMIT TRANSACTION statement. Once a transaction has been commit- ted, it can’t be rolled back. Although you can use a name in the COMMIT TRANSACTION statement, SQL Server makes no attempt to match this to a name in a BEGIN TRANSACTION state- ment. The name’s purely for your convenience in making your code more readable. 2627ch08.qxt 8/22/00 10:41 AM Page 270 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 271 COMMIT, with or without the optional keyword WORK, is exactly synonymous to COMMIT TRANSACTION with no transaction name. This form of the statement is ANSI SQL-92 compatible. ROLLBACK TRANSACTION ROLLBACK TRANSACTION also comes in two forms: ROLLBACK TRANS[ACTION] [transaction_name | @name_variable | savepoint_name | @savepoint_variable] ROLLBACK [WORK] ROLLBACK TRANSACTION throws away all changes since the most recent BEGIN TRANSACTION. Again, you can supply a transaction name as either a constant or a variable, but SQL Server ignores this name. You can also roll back part of a transaction by supplying a savepoint name. We’ll talk about savepoints in the next section. If a transaction is a distributed transaction (one that affects databases on multiple servers), you can’t roll back to a savepoint. ROLLBACK, with or without the optional WORK keyword, is the SQL-92 compli- ant form of the statement. However, in this form, you can’t roll back only one of a set of nested transactions. ROLLBACK WORK always rolls back to the outermost (first) transaction in a batch. WARNING ROLLBACK WORK rolls back all nested transactions and sets @@TRANCOUNT to zero. If you call ROLLBACK TRANSACTION as part of a trigger, subsequent SQL state- ments in the same batch are not executed. On the other hand, if you call ROLLBACK TRANSACTION in a stored procedure, subsequent SQL statements in the same batch are executed. SAVE TRANSACTION The SAVE TRANSACTION statement lets you partially commit a transaction, while still being able to roll back the rest of the transaction: SAVE TRANS[ACTION] {savepoint_name | @savepoint_variable} TRANSACTIONS Transact-SQL PART II 2627ch08.qxt 8/22/00 10:41 AM Page 271 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. CHAPTER 8 • ADVANCED TRANSACT-SQL 272 Note that when you issue SAVE TRANSACTION, you must name it. This name pro- vides a reference point for a subsequent COMMIT TRANSACTION or ROLLBACK TRANSACTION statement. An example will make the use of SAVE TRANSACTION more clear. Consider the following T-SQL batch: BEGIN TRANSACTION UPDATE titles SET price = 20.00 WHERE title_id = ‘TC7777’ SAVE TRANSACTION pricesaved UPDATE titles SET type = ‘potboiler’ WHERE title_id = ‘TC7777’ ROLLBACK TRANSACTION pricesaved COMMIT TRANSACTION In this case, the ROLLBACK TRANSACTION statement removes the effects of the update to the type column, while leaving the update to the price column ready to be committed. Then the COMMIT TRANSACTION statement commits the part of the transaction that wasn’t rolled back (in this case, the change to the price column). @@TRANCOUNT The @@TRANCOUNT system global variable tells you the number of nested transac- tions that are currently pending. If no transactions are pending, this variable will con- tain zero. This is useful for determining whether a trigger, for example, is executing in the middle of a transaction already started by a T-SQL batch. @@ERROR The @@ERROR system global variable holds the most recent error number from any T-SQL statement. Whenever a statement is executed that does not cause an error, this variable will contain zero. That is, it’s reset to zero every time you successfully execute a statement. So if you want to check at some later point whether a statement has caused an error, you need to save the value of @@ERROR to a local variable. A Transaction Example Let’s end this section with a more complex T-SQL batch that will illustrate the transaction-processing statements: DECLARE @price_err int, @type_err int BEGIN TRANSACTION 2627ch08.qxt 8/22/00 10:41 AM Page 272 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 273 UPDATE titles SET price = 20.00 WHERE title_id = ‘TC7777’ SET @price_err = @@ERROR SAVE TRANSACTION pricesaved UPDATE titles SET type = ‘potboiler’ WHERE title_id = ‘TC7777’ SET @type_err = @@ERROR IF @type_err <> 0 ROLLBACK TRANSACTION pricesaved IF @price_err = 0 AND @type_err = 0 BEGIN COMMIT TRANSACTION PRINT ‘Changes were successful’ END ELSE ROLLBACK TRANSACTION Here’s a blow-by-blow account of this batch: 1. The DECLARE statement sets up two local variables. 2. The BEGIN TRANSACTION statement starts a transaction. 3. The first UPDATE statement makes a change to the price column. 4. The first SET statement is used to save the value of @@ERROR so that you can check later whether the first UPDATE statement was successful. Note that this statement must immediately follow the UPDATE statement. 5. The SAVE TRANSACTION statement sets a savepoint. 6. The second UPDATE statement makes a change to the type column. 7. The second SET statement is used to save the value of @@ERROR so you can tell whether the second UPDATE statement succeeded. 8. If there was an error on the second UPDATE statement, the first ROLLBACK TRANSACTION statement undoes the transaction back to the savepoint. 9. If there are no errors at all, the transaction is committed, and a message is printed. Note the use of BEGIN and END to group two T-SQL statements into one logical statement. This is necessary because the IF statement refers only to the following statement. TRANSACTIONS Transact-SQL PART II 2627ch08.qxt 8/22/00 10:41 AM Page 273 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. CHAPTER 8 • ADVANCED TRANSACT-SQL 274 10. If there are any errors, the second ROLLBACK TRANSACTION statement undoes all of the work. Distributed Transactions So far, we’ve been discussing local transactions: those that make changes in a single database. SQL Server also supports distributed transactions: transactions that make changes to data stored in more than one database. These databases need not be SQL Server databases; they can be databases on other linked servers. NOTE For more information on linked servers, see Chapter 6. A distributed transaction can be managed in code using exactly the same SQL statements as you’d use for a local transaction. However, when you issue a COMMIT TRANSACTION on a distributed transaction, SQL Server automatically invokes a pro- tocol called two-phase commit (sometimes referred to as 2PC). In the first phase, SQL Server asks every database involved to prepare the transaction. The individual data- bases verify that they can commit the transaction and set aside all the resources nec- essary to do so. It’s only if every involved database tells SQL Server that it’s OK to commit the transaction that the second phase starts. In this phase, SQL Server tells every involved database to commit the transaction. If any of the databases involved are unable to commit the transaction, SQL Server tells all of the databases to roll back the transaction instead. Microsoft DTC Distributed transactions are managed by a SQL Server component called the Distrib- uted Transaction Coordinator (DTC). This is a separate service that’s installed at the same time as SQL Server. If you’re going to use distributed transactions, you should set this service to autostart. Figure 8.1 shows this service selected in the SQL Server Service Manager. 2627ch08.qxt 8/22/00 10:41 AM Page 274 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 275 FIGURE 8.1 Checking the status of the Microsoft DTC service BEGIN DISTRIBUTED TRANSACTION You can tell SQL Server explicitly to start a distributed transaction with the BEGIN DISTRIBUTED TRANSACTION statement: BEGIN DISTRIBUTED TRANS[ACTION] [transaction_name | @name_variable] The only difference between this statement and the regular BEGIN TRANSACTION statement is the inclusion of the DISTRIBUTED keyword. Local transactions are automatically escalated to distributed transactions if you change data on a remote server during the transaction. For example, if you execute an INSERT, UPDATE, or DELETE statement on a remote server, or call a remote stored procedure, while you’re in the midst of a transaction, that transaction will become a distributed transaction. Transaction Tips Transactions consume resources on the server. In particular, when you change data within a transaction, that data must be locked to ensure that it’s available if you com- mit the transaction. So, in general, you need to make transactions efficient to avoid causing problems for other users. Here are a few points to consider: • Don’t do anything that requires user interaction within a transaction, because this can cause locks to be held for a long time while the application is waiting for the user. • Don’t start transactions for a single SQL statement. • Change as little data as possible when in a transaction. TRANSACTIONS Transact-SQL PART II 2627ch08.qxt 8/22/00 10:41 AM Page 275 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. CHAPTER 8 • ADVANCED TRANSACT-SQL 276 • Don’t start a transaction while the user is browsing through data. Wait until they’re actually ready to change the data. • Keep transactions as short as possible. Rowset Functions Rowset functions are functions that return an object that can be used in place of a table in another SQL statement. For example, as you saw in Chapter 7, some rowset func- tions can be used to provide the rows to be inserted with an INSERT statement. There are five rowset functions in SQL Server 2000: • CONTAINSTABLE • FREETEXTTABLE • OPENQUERY • OPENROWSET • OPENDATASOURCE CONTAINSTABLE The CONTAINSTABLE statement lets you construct a virtual table from the results of a complex full-text search. This statement’s syntax is a bit more complicated than that of most of the statements we’ve examined so far: CONTAINSTABLE (table_name, {column_name | *}, ‘<search_condition>’ [,top_n]) <search_condition>::= { <generation_term> | <prefix_term> | <proximity_term> | <simple_term> | <weighted_term> } | {(<search_condition>) {AND | AND NOT | OR} <search_condition> […n] } 2627ch08.qxt 8/22/00 10:41 AM Page 276 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 277 <generation_term> ::= FORMSOF(INFLECTIONAL, <simple_term> [,…n]) <prefix_term> ::= {“word*” | “phrase*”} <proximity_term> ::= {<simple_term> | <prefix_term> } {{NEAR | ~} {<simple_term> | <prefix_term>}} […n] <simple_term> ::= word | “phrase” <weighted_term> ::= ISABOUT ( {{ <generation_term> | <prefix_term> | <proximity_term> | <simple_term> } [WEIGHT (weight_value)] } [,…n]) TIP You can use CONTAINSTABLE only on a table that’s been enabled for full-text indexing. For more on full-text indexing, see Chapter 6. If you work carefully through that syntax, you’ll see that the basic idea of CON- TAINSTABLE is to allow you to do a “fuzzy” search, which returns items that might not match entirely. Some further syntactical notes: • Using the asterisk (*) to specify columns tells CONTAINSTABLE to search all columns that have been registered for full-text searching, which might not be all the columns in the table. • Weight values are numbers between zero and one that specify how important each match is considered to be in the final virtual table. ROWSET FUNCTIONS Transact-SQL PART II 2627ch08.qxt 8/22/00 10:41 AM Page 277 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. CHAPTER 8 • ADVANCED TRANSACT-SQL 278 • You can limit the number of results returned by specifying an integer in the top_n parameter. This is useful when you’re searching a very large source table and want to see only the most important matches. The CONTAINSTABLE statement returns a virtual table containing two columns, always named KEY and RANK. For example, consider the following statement: SELECT * FROM CONTAINSTABLE(Products, ProductName, ‘ISABOUT(mix WEIGHT(.8), sauce WEIGHT(.2))’) Assuming that you’ve enabled the Product table in the Northwind sample database for full-text searching on the ProductName column, this statement returns the results shown in Figure 8.2. The ISABOUT search condition here specifies that results con- taining the word mix should be rated as more important than those containing the word sauce. FIGURE 8.2 Using CONTAINSTABLE to generate a virtual table The KEY column will always contain values from the column that you identified as the primary key to the full-text indexing service. To make this statement more useful, you’ll probably want to use this column to join back to the original table. Figure 8.3 shows the results of the following statement: SELECT ProductName, RANK FROM CONTAINSTABLE(Products, ProductName, ‘ISABOUT(mix WEIGHT(.8), sauce WEIGHT(.2))’) 2627ch08.qxt 8/22/00 10:41 AM Page 278 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 279 AS C INNER JOIN Products ON Products.ProductID = C.[KEY] FIGURE 8.3 Using CONTAINSTABLE joined to the original search table NOTE The virtual table needs to be aliased to be included in a join, and you must include the square brackets around the joining name because KEY is a SQL Server keyword. FREETEXTTABLE Like CONTAINSTABLE, FREETEXTTABLE generates a virtual table based on full-text indexing information. However, the syntax of FREETEXTTABLE is a good deal simpler: FREETEXTTABLE (table_name, {column_name | *}, ‘freetext’ [,top_n]) TIP You can use FREETEXTTABLE only on a table that’s been enabled for full-text indexing. For more on full-text indexing, see Chapter 6. ROWSET FUNCTIONS Transact-SQL PART II 2627ch08.qxt 8/22/00 10:41 AM Page 279 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... creating linked servers, see Chapter 6 Figure 8.5 shows in SQL Server Enterprise Manager that the MOOCOW server knows about a linked server named BIGREDBARN, which is also a Microsoft SQL Server If you connected to the BIGREDBARN server directly, you could run a query like the following: SELECT * FROM Northwind.dbo.Customers PA R T II Transact -SQL FIGURE 8.5 Inspecting properties for a linked server This... you store a SQL Server view, the server creates an execution plan for that view The execution plan is a list of the steps that SQL Server will take to get the results of the view This plan is based on statistical information that the server maintains about things such as the number of rows in each table, the number of unique indexes in each table, and so on Based on this information, SQL Server decides... CHAPTER 8 • ADVANCED TRANSACT -SQL • Retrieving metadata • Using optimizer hints At this point, you should know enough T -SQL to handle most of your querying needs Now it’s time to look at SQL Server from a different standpoint, by considering the objects that SQL Server stores rather than the data that those objects hold In the next chapter, we’ll start with a look at SQL Server Enterprise Manager Please... BIGREDBARN server has been incorporated as one of the parameters of the OPENQUERY statement OPENQUERY is the easiest tool that you can use to perform distributed queries using SQL Server By using OPENQUERY, you can join any number of tables from different data sources These data sources don’t even need to be SQL Server tables; as long as they’re data sources that you can represent as linked servers (basically,... it won’t reflect the actual number of rows in the cursor That’s because SQL Server might decide to fetch data into the cursor asynchronously, so that processing can continue while the cursor is still being populated SQL Server will fill a cursor asynchronously if the cursor is declared with the STATIC or KEYSET parameters and SQL Server estimates that the number of rows will be greater than a certain... the exact syntax SQL Server uses for full-text searches Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark 2627ch08.qxt 8/22/00 10:41 AM Page 281 ROWSET FUNCTIONS 281 OPENQUERY The OPENQUERY statement lets you use any query (SQL statement that returns rows) on a linked server to return a virtual table The syntax of OPENQUERY is as follows: OPENQUERY(linked _server, ‘query’)... • If the cursor was declared with SQL- 92 syntax without SCROLL, only NEXT is supported • If the cursor was declared with SQL- 92 syntax with SCROLL, all options are supported • If the cursor was declared with SQL Server syntax with FORWARD_ONLY or FAST_FORWARD, only NEXT is supported Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Transact -SQL 2627ch08.qxt 2627ch08.qxt 290... to remove this watermark Transact -SQL 2627ch08.qxt 2627ch08.qxt 290 8/22/00 10:41 AM Page 290 CHAPTER 8 • ADVANCED TRANSACT -SQL • If the cursor was declared with SQL Server syntax with DYNAMIC SCROLL, all options except ABSOLUTE are supported • If the cursor was declared with SQL Server syntax and doesn’t fall into one of the above two categories, all options are supported The @@FETCH_STATUS global variable... T -SQL statements? You can refer to Figure 8.9 to confirm your results Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Transact -SQL 2627ch08.qxt 2627ch08.qxt 294 8/22/00 10:41 AM Page 294 CHAPTER 8 • ADVANCED TRANSACT -SQL FIGURE 8.9 Running a batch in SQL Query Analyzer NOTE If you’re working with a client data-access library such as ADO, you may never need to deal with SQL. .. system tables are a set of tables that SQL Server uses to track information about users, databases, tables, replication tasks, and so on If SQL Server knows about a piece of information, it’s more than likely stored in a system table System tables break down into seven groups: • The master database contains a set of tables with information on databases, logins, servers, and other systemwide information . creating linked servers, see Chapter 6. Figure 8.5 shows in SQL Server Enterprise Manager that the MOOCOW server knows about a linked server named BIGREDBARN,. every involved database tells SQL Server that it’s OK to commit the transaction that the second phase starts. In this phase, SQL Server tells every involved