In this blog post we are going to discuss about Overriding Identity Fields.
For students new to the database world, it helps to begin thinking about ID fields in the context of larger organizations with lots of activity. A customer service department has a constant flow of activity and many representatives are entering data in the system simultaneously. The same is true for large billing departments. These are examples where an identity field helps to ensure the entities you care about get tracked properly. A CustomerID value that is automatically generated with each new record makes sure each new customer gets a unique number – even if you have many reps all entering data at the same time.
The CustomerID field is one we wouldn’t want accidentally duplicated, altered, or deleted. Similarly, the billing department would not want to have mistaken entries in the InvoiceID field. A missing InvoiceID could indicate a serious error (e.g., a customer wouldn’t get billed and JProCo wouldn’t get paid for that order) or even fraud. A duplicate InvoiceID value might result in a customer’s payment being applied to the wrong customer.
Identity fields help prevent these unwanted scenarios of ambiguity. For larger tables, like JProCo’s CurrentProducts, having ProductID as an identity field ensures each ProductID value will be unique and sequential. It also saves JProCo’s product managers from having to track down which ProductID to use each time they quickly need to add new products.
One hint about the identity property is to count the number of times we’ve used the words “large” and “active” in this section. For large tables where new records get added daily, the identity property saves time and helps enforce data integrity. But with smaller tables where records don’t often change, using the identity property to create your ID field is unnecessary and its automatic incrementing can make extra work for you.
Tables where records are frequently deleted also make poor candidates for the identity property. In our next example, we will see that an identity field’s ability to auto-increment and keep track of the next expected value can require extra maintenance tasks when fields are deleted. The example were going to use is the CurrentProducts table which has 480 records (see figure below).
Click on Images to see it in in original size.
There are exception cases when you will need to alter a value in the identity field. When training a new database user, you might temporarily allocate them a few empty invoice records to practice on. Later the practice records will be deleted, but you’ll want to make sure your next invoice numbers appears in proper sequence. The ProductID field is auto populated each time a record is created since it has an identity property set to count by 1.
Let’s step through an example and pretend we don’t already know that ProductID field is an identity field, so we can read the error message SQL Server generates when you attempt to enter an ID into the identity field. Note: There is a reason this example is inserting by position and not by name and we will get to that later.
Notice we tried to enter 481 instead of letting SQL pick the next value? This results in an error message.
When you remove the 481 value (ProductID) from the code, and then the insert statement works correctly.
Check to see you have ProductID 481 inserted at the “Yoga Mtn Getaway 5 Days”. Once verified, delete all the yoga products, in order to simulate the accidental deletion scenario.
If you run the insert statement again you won’t get ProductID 481. You will get 482. There is no 481 and there won’t be unless you take charge and put it there.
Our next goal it to re-insert this Yoga trip with a value of 481. To do our next step we need to temporarily set the IDENTITY_INSERT property to ON. We successfully ran the first command setting IDENTITY_INSERT to ON for the CurrentProducts table. We next attempted to run the INSERT statement for the three yoga records.
Click on Images to see it in in original size.
In the error message prompts us to include a column list whenever we manually insert records to a table with an identity field. In other words, when manually inserting records, we have to pass the values by name and not position.
SQL Server will allow you to utilize IDENTITY_INSERT with just one table at a time. After you’ve completed the needed work, it’s very important to reset the IDENTITY_INSERT back to OFF. Just to check it worked we ran a SELECT statement and confirm the yoga records show in the table and can see the last record has a ProductID of 481.
Note: If you want to setup the sample JProCo database on your system you can watch this video. For this post you will want to run the SQLQueriesChapter3.0Setup.sql script from Volume 2.
You need to explicitly insert a value into an identity field for the SalesInvoice table. What two things must you do in order for your insert statement to successfully execute? (Choose two)
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