Batches in Visual Studio .NET

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Batches
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A batch is a set of Transact-SQL statements that are sent to and executed by SQL Server as a single unit. The most important characteristic of a batch is that it is parsed and executed on the server as an undivided entity. In some cases, batches are created implicitly. For example, if you execute a set of Transact-SQL statements from Query Analyzer, the program will treat that set as one batch and do so invisibly:
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Insert Into Part (Make, Model, Type) Values ('Toshiba', 'Portege 7010CT', 'notebook') Insert Into Part (Make, Model, Type) Values ('Toshiba', 'Portege 7020CT', 'notebook') Insert Into Part (Make, Model, Type) Values ('Toshiba', 'Portege 7030CT', 'notebook')
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C h a p t e r 6 : C o m p o s i t e Tr a n s a c t - S Q L C o n s t r u c t s : B a t c h e s , S c r i p t s , a n d Tr a n s a c t i o n s
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Some tools, such as Query Analyzer, osql, and isql, use the Go command to divide Transact-SQL code into explicitly set batches. In the following example, the code for dropping a stored procedure is in one batch and the code for creating a new stored procedure is in another. The batch is explicitly created using the Go command.
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If Exists (Select * From sysobjects Where id = object_id(N'[dbo].[prPartList]') And OBJECTPROPERTY(id, N'IsProcedure') = 1) Drop Procedure [dbo].[prPartList] Go Create Procedure prPartList As Select * from Part Return 0 Go
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In Query Analyzer, you can highlight part of the code and execute it. Query Analyzer treats the selected piece of code as a batch and sends it to the server and ignores the rest of the code (see Figure 6-1).
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Figure 6-1
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In other utilities and development environments, batches may be divided in some other manner. In ADO, OLEDB, ODBC, and DB-Library, each command string prepared for execution (in the respective object or function) is treated as one batch.
Using Batches
Batches reduce the time and processing associated with transferring statements from client to server, as well as that associated with parsing, compiling, and executing Transact-SQL statements. If you need to execute a set of 100 insert commands against a database, it is preferable to group them in one batch rather than send them to the server as 100 separate statements. The overhead involved in sending 100 separate statements and receiving 100 separate results is very high. Network traffic will be increased unnecessarily, and the whole operation will be slower for the user.
Batches and Errors
The fact that the batch is compiled as an undivided entity has interesting implications for statements that contain syntax errors. Results will vary according to whether the syntax error occurs in a statement or in the name of a database object. If you create a batch that includes a statement containing a syntax error, the whole batch will fail to execute. Consider the following batch:
Insert into Part (Make, Model, Type) Values ('Toshiba', 'Portege 7020CT', 'Notebook') Selec * from Part
It consists of two commands, the second of which contains a syntax error a missing letter in the Select keyword. If you execute this batch in Query Analyzer, SQL Server will not compile or execute it but will return the following error:
Server: Msg 170, Level 15, State 1, Line 3 Line 3: Incorrect syntax near 'Selec'
If you make a typo in the name of the database object (for instance, in a table or column name), the situation is very different. Note that the name of the table in the following Insert statement is incorrect:
Insert into art (Make, Model, Type) Values ('Toshiba', 'Portege 7020CT', 'Notebook') Select * from Part
C h a p t e r 6 : C o m p o s i t e Tr a n s a c t - S Q L C o n s t r u c t s : B a t c h e s , S c r i p t s , a n d Tr a n s a c t i o n s
In this example, the application will notice an error and stop execution as soon as it encounters it:
Server: Msg 208, Level 16, State 1, Line 1 Invalid object name 'art'.
SQL Server executes the batch in three steps: it parses, compiles, and then executes. In the first phase, SQL Server verifies batch syntax. It focuses on the sequence of keywords, operators, and identifiers. The first batch used a statement with an error in a keyword. SQL Server picked up the error during the parsing phase. The error in the second batch (an invalid object name) was picked up during execution. To further demonstrate this fact, let s investigate the following example, where the error is in the second statement:
Insert into Part (Make, Model, Type) Values ('Toshiba', 'Portege 7020CT', 'Notebook') Select * from art
In this case, the application behaves differently:
(1 row(s) affected) Server: Msg 208, Level 16, State 1, Line 1 Invalid object name 'art'.
Both commands are parsed and compiled, then the first command is executed, and finally the second command is canceled. Users with experience of earlier versions of Microsoft SQL Server remember that such a scenario would produce very different results in those earlier versions. Microsoft SQL Server 2000 supports deferred name resolution (actually introduced in SQL Server 7.0). Deferred name resolution allows the server to compile TransactSQL statements even when dependent objects do not yet exist in the database. This feature can prove to be very useful when you are creating or transferring objects from one database or server to another. You do not have to worry about dependencies and the order in which objects are created. Unfortunately, the introduction of this feature also has some strange secondary effects. In the case of the last example: The server has successfully compiled a batch, since the name resolution is not part of the compilation. The first command was executed without a problem. When a problem was encountered in the second command, the server canceled all further processing and returned a runtime error.
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