The Close Statement in VS .NET

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The Close Statement
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The Close statement closes an open cursor, releases the current recordset, and releases locks on rows held by the cursor:
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Close { { [Global] cursor_name } | cursor_variable_name }
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This statement must be executed on an opened cursor. If the cursor has just been declared, SQL Server will report an error.
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The Deallocate Statement
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After the Close statement, the structure of the cursor is still in place. It is possible to open it again. If you do not plan to use it any more, you should remove the structure as well, by using the Deallocate statement:
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Deallocate { { [Global] cursor_name } | @cursor_variable_name}
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Problems with Cursors
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Cursors are a valuable but dangerous tool. Their curse is precisely the problem they are designed to solve the differences between the relational nature of database systems and the record-based nature of client applications.
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First of all, cursors are procedural and thus contradict the basic idea behind the SQL language that is, to define what is needed in a result, not how to get it. Performance penalties are an even larger problem. Regular SQL statements are set-oriented and much faster. Some types of cursors lock records in the database and prevent other users from changing them. Other types of cursors create an additional copy of all records and then work with them. Both approaches have performance implications. Client-side cursors and API server cursors are also not the most efficient way to transfer information between server and client. It is much faster to use a fire hose cursor, which is actually not a cursor at all. You can find more details about fire hose cursors in Hitchhiker s Guide to Visual Basic and SQL Server, 6th edition, by William Vaughn (Microsoft Press, 1998).
The Justified Uses of Cursors
The rule of thumb is to avoid the use of cursors whenever possible. However, in some cases, such avoidance is not possible. Cursors can be used to perform operations that cannot be performed using set-oriented statements. It is acceptable to use cursors to perform processing based on statements, stored procedures, and extended stored procedures, which are designed to work with one item at a time. For example, the sp_addrolemember system stored procedure is designed to set an existing user account as a member of the SQL Server role. If you can list users that need to be assigned to a role, you can loop through them (using a cursor) and execute the system stored procedure for each of them. Excessive processing based on a single row (for example, business logic implemented in the form of an extended stored procedure) can also be implemented using a cursor. If you implement such a loop in a stored procedure instead of in a client application, you can reduce network traffic considerably. Another example could be the export of a group of tables from a database to text files using bcp. The bcp utility is a command-prompt program that can work with one table at a time. To use it within a stored procedure, you need to execute it using the xp_cmdshell extended stored procedure, which can run just one command at a time:
Create Procedure prBcpOutTables --loop through tables and export them to text fields @debug int = 0 As Declare @chvTable varchar(128), @chvCommand varchar(255)
Declare @curTables Cursor
C h a p t e r 4 : B a s i c Tr a n s a c t - S Q L P r o g r a m m i n g C o n s t r u c t s
-- get all USER-DEFINED tables from current database Set @curTables = Cursor FOR select name from sysobjects where xType = 'U' Open @curTables -- get first table Fetch Next From @curTables Into @chvTable -- if we successfully read the current record While (@@fetch_status = 0) Begin -- assemble DOS command for exporting table Set @chvCommand = 'bcp "Asset..[' + @chvTable + ']" out C:\sql7\backup\' + @chvTable + '.txt -c -q -Sdejan -Usa -Pdejan' -- during test just display command If @debug <> 0 Select @chvCommand chvCommand -- in production execute DOS command and export table If @debug = 0 Execute xp_cmdshell @chvCommand, NO_OUTPUT Fetch Next From @curTables Into @chvTable End Close @curTables Deallocate @curTables Return 0
If you execute this stored procedure (without specifying the @debug parameter), SQL Server will execute the following sequence of command-prompt commands to export tables:
bcp "Asset..[AcquisitionType]" out C:\sql7\backup\AcquisitionType.txt -c -q -Sdejan -Usa -Pdejan bcp "Asset..[MyEquipment]" out C:\sql7\backup\MyEquipment.txt -c -q
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