c# datamatrix open source Figure 8-46. Updating a column with internal data conversion in Visual C#

Drawer Data Matrix 2d barcode in Visual C# Figure 8-46. Updating a column with internal data conversion

Figure 8-46. Updating a column with internal data conversion
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CHAPTER 8 WORKING WITH THE DATA
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11. However, in this next example, the data type that SQL Server will come up with is a numeric data type. When we try to alter an integer-based data type, bigint, with this value, which we can find in the AddressId column, the UPDATE does not take place. Enter the following code: DECLARE @WrongDataType VARCHAR(20) SET @WrongDataType = '2.0' UPDATE CustomerDetails.Customers SET AddressId = @WrongDataType WHERE CustomerId = 1 12. Now execute the code. Notice when we do that SQL Server generates an error message informing you of the problem. Hence, never leave data conversions to SQL Server to perform. Try to get the same data type updating the same data type. We look at how to convert data within 12. Msg 8114, Level 16, State 5, Line 3 Error converting data type varchar to bigint. Updating data can be very straightforward, as the preceding examples have demonstrated. Where at all possible, either use a unique identifier, for example, the CustomerId, when trying to find a customer, rather than a name. There can be multiple rows for the same name or other type of criteria, but by using the unique identifier, you can be sure of using the right record every time. To place this in a production scenario, we would have a Windows-based graphical system that would allow you to find details of customers by their name, address, or account number. Once you found the right customer, instead of keeping those details to find other related records, keep a record of the unique identifier value instead. Getting back to the UPDATE command and how it works, first of all SQL Server will filter out from the table the first record that meets the criteria of the WHERE statement. The data modifications are then made, and SQL Server moves on to try to find the second row matching the WHERE statement. This process is repeated until all the rows that meet the WHERE condition are modified. Therefore, if using a unique identifier, SQL Server will only update one row, but the WHERE statement looks for rows that have a CustomerLastName of McGlynn, in which case multiple rows could be updated. So choose your row selection criteria for updates carefully. But what if you didn t want the update to occur immediately There will be times when you will want to perform an update, and then check that the update is correct before finally committing the changes to the table. Or when doing the update, you want to check for errors or unexpected data updates. This is where transactions come in, and these are covered next.
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Transactions
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A transaction is a method through which developers can define a unit of work logically or physically that, when it completes, leaves the database in a consistent state. A transaction forms a single unit of work, which must pass the ACID test before it can be classified as a transaction. The ACID test is an acronym for Atomicity, Consistency, Isolation, and Durability:
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CHAPTER 8 WORKING WITH THE DATA
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Atomicity: In its simplest form, all data modifications within the transaction must be both accepted and inserted successfully into the database, or none of the modifications will be performed. Consistency: Once the data has been successfully applied, or rolled back to the original state, all the data must remain in a consistent state, and the data must still maintain its integrity. Isolation: Any modification in one transaction must be isolated from any modifications in any other transaction. Any transaction should see data from any other transaction either in its original state or once the second transaction has completed. It is impossible to see the data in an intermediate state. Durability: Once a transaction has finished, all data modifications are in place and can only be modified by another transaction or unit of work. Any system failure (hardware or software) will not remove any changes applied. Transactions within a database are a very important topic, but also one that requires a great deal of understanding. This chapter covers the basics of transactions only. To really do justice to this area, we would have to deal with some very complex and in-depth scenarios, covering all manner of areas such as triggers, nesting transactions, and transaction logging, which is beyond the scope of this book. A transaction can be placed around any data manipulation, whether it is an update, insertion, or deletion, and can cater to one row or many rows, and also many different commands. There is no need to place a transaction around a SELECT statement unless you are doing a SELECT...INTO, which is of course modifying data. This is because a transaction is only required when data manipulation occurs such that changes will either be committed to the table or discarded. A transaction could cover several UPDATE, DELETE, or INSERT commands, or indeed a mixture of all three. However, there is one very large warning that goes with using transactions.
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Caution Be aware that when creating a transaction, you will be keeping a hold on the whole table, pages of data, or specific rows of information in question, and depending upon how your SQL Server database is set up to lock data during updates, you could be stopping others from updating any information, and you could even cause a deadlock, also known as a deadly embrace. If a deadlock occurs, SQL Server chooses one of the deadlocks and kills the process; there is no way of knowing which process SQL Server will select.
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A deadlock is where two separate data manipulations, in different transactions, are being performed at the same time. However, each transaction is waiting for the other to finish the update before it can complete its update. Neither manipulation can be completed because each is waiting for the other to finish. A deadlock occurs, and it can (and will) lock the tables and database in question. So, for example, transaction 1 is updating the customers table
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