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Oracle® Application Express Application Builder User's Guide
Release 4.1

Part Number E21674-05
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Understanding Page Computations

Use page computations to assign a value to an identified item when a page is submitted or displayed. You can also use application-level computations to assign values to items. Most page-level computations populate page items. In contrast, most application-level computations populate application items.

Topics:

Creating a Page Computation

A page computation assigns a value to an identified item when a page is displayed or submitted (rendered and processed).You create a page computation by running the Create Page Computation Wizard. For each computation, specify the item for which you are creating the computation and a computation type.

To create a page computation:

  1. Navigate to the appropriate Page Definition. See "Accessing the Page Definition".

  2. Access the Create Computation Wizard:

    • Tree view - Under the appropriate page rendering event (such as Before Header, After Header and so on), locate Computations. Right-click and select Create.

    • Component view - Under Computations, click the Create icon.

  3. For Item Type, select where the computation will execute and click Next. Location options include:

    • Item on Another Page

    • Item on This Page

    • Application Level Item

  4. For Item, select the item and computation point at which you would like to perform the computation and click Next:

    1. Compute Item - Select the item the computation will update.

    2. Sequence - Select the order of evaluation.

    3. Computation Point - Select the point at which the computation executes. The computation point On New Instance executes the computation when a new session (or instance) is generated.

    4. Computation Type - Select the method of computation you want to create.

  5. In Computation, enter a computation that corresponds to the selected computation type and click Next.

  6. On Condition, you can choose to make the computation conditional. To make a computation conditional, make a selection from the Condition Type list and enter text in the expression fields.

  7. Click Create.

Understanding Computation Points and Computation Syntax

A good example of using computations can be illustrated by a page containing form fields for entering phone numbers. In this example, the phone number is stored in one database column; however, the data entry form breaks the phone number into three components: area code, prefix, and line number. In this example, the page items are called P10_AREA_CODE, P10_PREFIX, and P10_LINE_NUMBER.

Next, suppose you need to combine the values stored in these items into a single string. You could accomplish this by using an After Submit computation and store the combined values in an item called P10_PHONE_NUMBER.

To create a computation to store the combined values of P10_AREA_CODE, P10_PREFIX, and P10_LINE_NUMBER in new items:

  1. Navigate to the appropriate Page Definition. See "Accessing the Page Definition".

  2. Create an item named P10_PHONE_NUMBER to store the combined values of P10_AREA_CODE, P10_PREFIX, and P10_LINE_NUMBER. See "Differences Between Page Items and Application Items".

  3. Access the Create Computation Wizard:

    • Tree view - Under the appropriate event, locate Computations. Right-click and select Create.

    • Component view - Under Computations, click the Create icon.

  4. For Item Location, select Item on this Page and click Next.

  5. For Computation, select P10_PHONE_NUMBER.

  6. For Sequence, select the order of evaluation.

  7. For Computation, you have the option of creating one of the following computation types:

    1. Static Assignment:

      • For Computation Type, select Static Assignment and click Next.

      • Enter the following computation:

        (&P10_AREA_CODE.) &P10_PREFIX.-&P10_LINE_NUMBER.
        
      • Click Next.

    2. PL/SQL Function Body:

      • For Computation Type, select PL/SQL Function Body and click Next.

      • Enter the following computation:

        DECLARE
        l_return_value  VARCHAR2(300) DEFAULT NULL;
        BEGIN
            l_return_value :=
        '('||:P10_AREA_CODE||')'||:P10_PREFIX||'-'||:P10_LINE_NUMBER;
        RETURN l_return_value;
        END;
        
      • Click Next.

    3. SQL Query (return colon separated value):

      • For Computation Type, select SQL Query (return colon separated value) and click Next.

      • Enter the following computation:

        SELECT '('||:P10_AREA_CODE||')'||:P10_PREFIX||'-'||:P10_LINE_NUMBER FROM DUAL
        
      • Click Next.

    4. PLSQL Expression:

      • For Computation Type, select PLSQL Expression and click Next.

      • Enter the following computation:

        '('||:P10_AREA_CODE||')'||:P10_PREFIX||'-'||:P10_LINE_NUMBER
        
      • Click Next.

  8. Click Create.

Editing Page Computation Attributes

Once you create a computation, you can edit it on the Edit Page Computation page.

To edit a page computation:

  1. Navigate to the appropriate Page Definition. See "Accessing the Page Definition".

  2. Access the Edit Page Computation page:

    • Tree view - Under the appropriate event, locate Computations. Right-click the computation name and select Edit.

    • Component view - Under Computations, select the computation name.

    The Edit Page Computation page appears.

  3. Edit the appropriate attributes.

  4. Click Apply Changes.

Editing the Computation Point and Source

You control when a computation executes under the Computation Point attributes by specifying a sequence and a computation point. The computation point On New Instance executes the computation when a new session (or instance) is generated.

Under Source, enter an expression or query to compute an item's value. In the event a computation fails, you can optionally define an error message in the Computation Error Message field.

Creating Conditional Computations

You can make a computation conditional by making a selection from the Condition Type list and entering text in the expression fields.