c# datamatrix open source Deleting Data in C#

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Deleting Data
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Deleting data can be considered very straightforward, especially compared to all of the other data manipulation functions covered previously, particularly transactions and basic SQL. However, mistakes made when deleting data are very hard to recover from. Therefore, you must treat deleting data with the greatest of care and attention to detail, and especially test any joins and filtering via a SELECT statement before running the delete operation. Deleting data without the use of a transaction is almost a final act: the only way to get the data back is to reenter it, restore it from a backup, or retrieve the data from any audit tables that had the data stored in them when the data was created. Deleting data is not like using the recycle bin on a Windows machine: unless the data is within a transaction, it is lost. Keep in mind that even if you use a transaction, the data will be lost once the transaction is committed. That s why it s very important to back up your database before running any major data modifications. This section of the chapter will demonstrate the DELETE T-SQL syntax and then show how to use this within Query Editor. It is also possible to delete records from the results pane within SQL Server Management Studio, which will also be demonstrated. However, what about when you want to remove all the records within a table, especially when there could be thousands of records to remove You will find that the DELETE command takes a very long time to run, as each row to delete is logged in the transaction log, thus allowing transactions to be rolled back. Luckily, there is a command for this scenario, called TRUNCATE, which is covered in the section Truncating a Table later in the chapter. However, caution should be exercised when using this command, and you ll see why later. First of all, it is necessary to learn the simple syntax for the DELETE command for deleting records from a table. Really, things don t come much simpler than this.
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DELETE Syntax
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The DELETE command is very short and sweet. To run the command, simply state the table you wish to delete records from, as shown here: DELETE [FROM] tablename WHERE where_condition The FROM condition is optional, so your syntax could easily read DELETE tablename WHERE where_condition
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CHAPTER 8 WORKING WITH THE DATA
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There is nothing within this command that has not been covered in other chapters. The only area that really needs to be mentioned is that records can only be deleted from one table at a time, although when looking for rows to delete, you can join to several tables, as you can with SELECT and UPDATE. Now that you ve seen the DELETE syntax, let s dive right in with an example.
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Using the DELETE Statement
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Recall we created a table with the SELECT INTO command called CustTemp. Rather than delete data from the main tables created so far, we ll use this temporary table in this section of the book. We ll use transactions a great deal here to avoid having to keep inserting data back into the table. It s a good idea to use transactions for any type of table modification in your application. Imagine that you re at the ATM and you are transferring money from your savings account to your checking account. During that process, a transaction built up of many actions is used to make sure that your money doesn t credit one system and not the other. If an error occurs, the entire transaction rolls back, and no money will move between the accounts. Let s take a look at what happens if you were to run this statement: BEGIN TRAN DELETE CustTemp When this code runs, SQL Server opens a transaction and then tentatively deletes all the records from the CustTemp table. The records are not actually deleted until a COMMIT TRAN statement is issued. In the interim, though, SQL Server will place a lock on the rows of the table, or if this was a much larger table, SQL Server may decide that a table lock (locking the whole table to prevent other modifications) is better. Because of this lock, all users trying to modify data from this table will have to wait until a COMMIT TRAN or ROLLBACK TRAN statement has been issued and completed. If one is never issued, users will be blocked. This problem is one of a number of issues frequently encountered in applications when analyzing performance issues. Therefore, never have a BEGIN TRAN without a COMMIT TRAN or ROLLBACK TRAN. So, time to start deleting records.
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