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SQL Server 2000 Stored Procedure Programming
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-- some work Select @Counter -- this line is meaningless: -- we need to do something to demonstrate loop set @Counter = @Counter + 1 GoTo LOOP end
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The point of this example is not merely to replace the GoTo statement in a mechanical manner. The point is that use of the While statement produces code that is much easier to read. Thus, replacing GoTo with While is a change for the better. Some database administrators base their error-handling practices on the use of the GoTo statement. There is an example of this type of code in the stored procedure prCloseLease shown in the previous section. This solution is not a perfect one. You will see several other error-handling solutions in 7.
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Scheduled Execution the WaitFor Statement
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There are two ways to schedule the execution of a batch or stored procedure in SQL Server. One way is based on the use of SQL Server Agent (a tool formerly known as Task Scheduler). The other way is to use the WaitFor statement. The WaitFor statement allows the developer to specify the time when, or a time interval after which, the remaining Transact-SQL statements will be executed:
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WaitFor {Delay 'time' | Time 'time'}
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There are two variants to this statement. One specifies the delay (time interval) that must pass before the execution can continue. The time interval specified as a parameter of the statement must be less than 24 hours. In the following example, the server will pause for one minute before displaying the list of Equipment:
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WaitFor Delay '00:01:00' Select * from Equipment
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The other variant is more significant. It allows the developer to schedule a time when the execution is to continue. The following example runs a full database backup at 11:00 P.M.:
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Basic Transact-SQL Programming Constructs
WaitFor Time '23:00' Backup Database Asset To Asset_bkp
There is one problem with this Transact-SQL statement. The connection remains blocked while the server waits to execute the statement. Therefore, it is much better to use SQL Server Agent than the WaitFor statement to schedule jobs.
CURSORS
Relational databases are designed to work with sets of information (records). In fact, the purpose of the Select statement, as the most important statement in SQL, is to define a set of records. In contrast, end-user applications display information to the user record by record (or maybe in small batches). To close the gap between these conflicting requirements, RDBMS architects have invented a new class of programming constructs cursors. Many types of cursors are implemented in various environments using different syntax, but all cursors work in a similar fashion: 1. A cursor first has to be defined and its features have to be set. 2. The cursor must be populated. 3. The cursor then has to be positioned (scrolled) to a record or block of records that need to be retrieved (fetched). 4. Information from one or more current records is fetched, and then some modification can be performed or some action can be initiated based on the fetched information. 5. Optionally, steps 3 and 4 are repeated. 6. Finally, the cursor must be closed and resources released. Cursors can be used on both server and client sides. SQL Server and the APIs for accessing database information (OLE DB, ODBC, DB-Library) all include sets of functions for processing cursors. SQL Server supports three classes of cursors:
w s v
Client cursors API Server cursors Transact-SQL cursors
SQL Server 2000 Stored Procedure Programming
The major difference between Transact-SQL cursors and other types of cursors is their purpose. Transact-SQL cursors are used from stored procedures, batches, functions, or triggers to repeat custom processing for each row of the cursor. Other kinds of cursors are designed to access database information from the client application. We will review only Transact-SQL cursors.
Transact-SQL Cursors
Processing in Transact-SQL cursors has to be performed in the following steps: 1. Use the Declare Cursor statement to create the cursor based on the Select statement. 2. Use the Open statement to populate the cursor. 3. Use the Fetch statement to change the current record in the cursor and to store values into local variables. 4. Do something with the retrieved information. 5. If needed, repeat steps 3 and 4. 6. Close the cursor. Most of the resources (memory, locks ) will be released. 7. Deallocate the cursor.
NOTE: Transact-SQL cursors do not support processing blocks of records. Only one record can be fetched at a time.
It is best to show this process through an example. We will rewrite the stored procedure that we used to illustrate the use of the While statement. The purpose of this stored procedure is to collect the properties of a specified asset and return them in delimited format (Property = Value Unit;). The final result should look like this:
CPU=Pentium II;RAM=64 MB;HDD=6.4 GB;Resolution=1024x768;Weight=2 kg;
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