Working with Computed Columns Cby Scott Mitchell. Download Code or Download PDFWhen creating a database table, Microsoft SQL Server allows you to define a computed column whose value is calculated from an expression that usually references other values in the same database record. Such values are read only at the database, which requires special considerations when working with Table. Adapters. In this tutorial we learn how to meet the challenges posed by computed columns. ExpressionWeb/importwiz_frontpgext.jpg' alt='Microsoft Expression Web 4 Step Step Pdf Reader' title='Microsoft Expression Web 4 Step Step Pdf Reader' />Oracle Technology Network is the ultimate, complete, and authoritative source of technical information and learning about Java. 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Search-for-a-Word-or-Phrase-in-a-PDF-Document-Step-2-Version-4.jpg/aid17824-v4-728px-Search-for-a-Word-or-Phrase-in-a-PDF-Document-Step-2-Version-4.jpg' alt='Microsoft Expression Web 4 Step Step Pdf Reader' title='Microsoft Expression Web 4 Step Step Pdf Reader' />Introduction. Microsoft SQL Server allows for computed columns, which are columns whose values are calculated from an expression that usually references the values from other columns in the same table. As an example, a time tracking data model might have a table named Service. Log with columns including Service. Performed, Employee. ID, Rate, and Duration, among others. While the amount due per service item being the rate multiplied by the duration could be calculated through a web page or other programmatic interface, it might be handy to include a column in the Service. Log table named Amount. Due that reported this information. This column could be created as a normal column, but it would need to be updated anytime the Rate or Duration column values changed. A better approach would be to make the Amount. Due column a computed column using the expression Rate Duration. Doing so would cause SQL Server to automatically calculate the Amount. Due column value whenever it was referenced in a query. Since a computed column s value is determined by an expression, such columns are read only and therefore cannot have values assigned to them in INSERT or UPDATE statements. However, when computed columns are part of the main query for a Table. Adapter that uses ad hoc SQL statements, they are automatically included in the auto generated INSERT and UPDATE statements. Consequently, the Table. Adapter s INSERT and UPDATE queries and Insert. Command and Update. Command properties must be updated to remove references to any computed columns. One challenge of using computed columns with a Table. Adapter that uses ad hoc SQL statements is that the Table. Adapter s INSERT and UPDATE queries are automatically regenerated any time the Table. Adapter Configuration wizard is completed. Therefore, the computed columns manually removed from the INSERT and UPDATE queries will reappear if the wizard is re run. Although Table. Adapters that use stored procedures don t suffer from this brittleness, they do have their own quirks that we will address in Step 3. In this tutorial we will add a computed column to the Suppliers table in the Northwind database and then create a corresponding Table. Adapter to work with this table and its computed column. We will have our Table. Adapter use stored procedures instead of ad hoc SQL statements so that our customizations aren t lost when the Table. Adapter Configuration wizard is used. Let s get started Step 1 Adding a Computed Column to the. Suppliers. Table. The Northwind database does not have any computed columns so we will need to add one ourselves. For this tutorial let s add a computed column to the Suppliers table called Full. Contact. Name that returns the contact s name, title, and the company they work for in the following format Contact. Name Contact. Title, Company. Name. This computed column might be used in reports when displaying information about suppliers. Start by opening the Suppliers table definition by right clicking on the Suppliers table in the Server Explorer and choosing Open Table Definition from the context menu. This will display the columns of the table and their properties, such as their data type, whether they allow NULL s, and so forth. To add a computed column, start by typing in the name of the column into the table definition. Next, enter its expression into the Formula textbox under the Computed Column Specification section in the Column Properties window see Figure 1. Name the computed column Full. Contact. Name and use the following expression Contact. Name CASE WHEN Contact. Title IS NOT NULL THEN. Contact. Title, ELSE END Company. Name. Note that strings can be concatenated in SQL using the operator. The CASE statement can be used like a conditional in a traditional programming language. In the above expression the CASE statement can be read as If Contact. Title is not NULL then output the Contact. Title value concatenated with a comma, otherwise emit nothing. For more on the usefulness of the CASE statement, see The Power of SQL CASE Statements. Note. Instead of using a CASE statement here, we could have alternatively used ISNULLContact. Title,. ISNULLcheck. Expression, replacement. Value returns check. Expression if it is non NULL, otherwise it returns replacement. Value. While either ISNULL or CASE will work in this instance, there are more intricate scenarios where the flexibility of the CASE statement cannot be matched by ISNULL. After adding this computed column your screen should look like the screen shot in Figure 1. Figure 1 Add a Computed Column Named Full. Contact. Name to the Suppliers Table Click to view full size imageAfter naming the computed column and entering its expression, save the changes to the table by clicking the Save icon in the toolbar, by hitting CtrlS, or by going to the File menu and choosing Save Suppliers. Saving the table should refresh the Server Explorer, including the just added column in the Suppliers table s column list. Installing Air Fuel Ratio Sensor more. Furthermore, the expression entered into the Formula textbox will automatically adjust to an equivalent expression that strips unnecessary whitespace, surrounds column names with brackets, and includes parentheses to more explicitly show the order of operations Contact. Name case when Contact. Title IS NOT NULL. Contact. Title, else endCompany. Name. For more information on computed columns in Microsoft SQL Server, refer to the technical documentation. Also check out the How to Specify Computed Columns for a step by step walkthrough of creating computed columns. Note. By default, computed columns are not physically stored in the table but are instead recalculated each time they are referenced in a query. By checking the Is Persisted checkbox, however, you can instruct SQL Server to physically store the computed column in the table. Doing so allows an index to be created on the computed column, which can improve the performance of queries that use the computed column value in their WHERE clauses. See Creating Indexes on Computed Columns for more information. Step 2 Viewing the Computed Column s Values. Before we start work on the Data Access Layer, let s take a minute to view the Full. Contact. Name values. From the Server Explorer, right click on the Suppliers table name and choose New Query from the context menu. This will bring up a Query window that prompts us to choose what tables to include in the query. Add the Suppliers table and click Close. Next, check the Company. Name, Contact. Name, Contact. Title, and Full. Contact. Name columns from the Suppliers table. Finally, click the red exclamation point icon in the Toolbar to execute the query and view the results. As Figure 2 shows, the results include Full. Contact. Name, which lists the Company. Name, Contact. Name, and Contact. Title columns using the format ldquo Contact. Name Contact. Title, Company. Name. Figure 2 The Full. Contact. Name Uses the Format Contact. Name Contact. Title, Company. HP Touch. Pad Needs 6 to 8 Weeks for Additional Shipments. Hewlett Packard will apparently need close to two months to start fulfilling backorders for the temporarily revived Touch. Pad tablet. It will take 6 8 weeks to build enough HP Touch. Pads to meet our current commitments, during which time your order will then ship from this stock with free ground shipping, read an email sent to customers and reprinted in a Sept. Precentral. net blog. You will receive a shipping notification with a tracking number once your order has shipped. That would place the new Touch. Pads in consumers hands sometime in either late October or early November. The reduced price devices are not returnable, according to the email. HP originally acquired web. OS as part of its takeover of Palm in 2. The manufacturer originally had big plans for loading the operating system onto a variety of devices, including tablets, smartphones, desktops and laptops. However, sales of its Touch. Pad proved anemic, and HP made the decision to end the tablets life after a mere six weeks on the market. In order to clear out inventory, the manufacturer sliced the starting price to 9. In the wake of that, HP made the decision to revive the line for a limited time. In addition, HP plans on dividing its web. OS arm into two separate units reporting to different areas of the company, according to two leaked memos that have made their way onto the Web. The web. OS software assets will find their way into the arms, however welcoming, of its Office of Strategy and Technology. The other parts of the web. OS corporate infrastructure, presumably including its hardware interests, will continue as part of the Personal Systems Group, which manufactures HPs PCs, and which will presumably be spun off into its own entity under the terms of the companys new strategy. We have decided that well be most effective in these efforts by having the teams in web. OS software engineering, worldwide developer relations and web. OS software product marketing join the Office of Strategy and Technology, Todd Bradley, executive vice president of HPs Personal Systems Group, wrote in an email circulated to the web. OS developer team and also leaked onto Precentral. The remainder of the web. OS team, under Stephen De. Witt, will continue to report into PSG. According to at least one analyst, flooding the market with additional Touch. Pad devices could have significant benefits for HP going forward. A larger installed base of Touch. Pad and web. OS devices should increase the value of web. OS in a potential sale, Sterne Agee analyst Shaw Wu wrote in a research note widely circulated on Barrons and other financial Websites. We believe logical buyers may include Samsung Electronics, Research In Motion, HTC, Amazon. Facebook, Sony, Microsoft and others. Follow Nicholas Kolakowski on Twitter.