PRE-QUERY Trigger in Oracle Forms

In Oracle Forms, developers often need to control how queries are executed on blocks and items. One of the most useful triggers for this purpose is the PRE-QUERY Trigger. This trigger allows you to modify or enhance query conditions dynamically before Oracle Forms actually fetches the data. In this article, you will learn what the PRE-QUERY Trigger is, how it works, why it is important, examples of its usage, and best practices for implementing it correctly.

What is the PRE-QUERY Trigger?

The PRE-QUERY Trigger is a block-level trigger in Oracle Forms. It fires immediately before a query is executed on a block. This gives developers the opportunity to customize the query by changing WHERE clause conditions or assigning values to block items before the query runs.

Key characteristics:

• It is defined at the block level.
• Fires before data is retrieved from the database.
• Allows developers to modify query conditions dynamically.
• Useful for adding security checks, default filters, or user-based restrictions.

Purpose of the PRE-QUERY Trigger

The main purpose of the PRE-QUERY Trigger is to control the type of data that is fetched by the query. Instead of relying solely on user-entered values, developers can add conditions or filters in the background.

Some common purposes include:

• Restricting data by user role (e.g., showing only employees of a specific department).
• Adding default conditions so users do not have to type them.
• Enhancing security by preventing access to sensitive data.
• Dynamically constructing WHERE clauses based on runtime variables.
• Improving performance by reducing the amount of unnecessary data fetched.

When Does the PRE-QUERY Trigger Fire?

The PRE-QUERY Trigger fires whenever a query is about to execute on a block. This can happen in the following scenarios:

1. When the user presses the Execute Query button.
2. When a query is executed programmatically using the EXECUTE_QUERY built-in.
3. When the block is entered and a query is triggered automatically (depending on form logic).

In every case, PRE-QUERY executes before the actual SQL SELECT statement runs against the database.

Difference Between PRE-QUERY and POST-QUERY

To avoid confusion, let us clarify the difference between the two commonly used triggers:

• PRE-QUERY Trigger: Fires before the query executes. Used to modify or add query conditions.
• POST-QUERY Trigger: Fires after each record is fetched. Used to perform additional actions on retrieved data, such as formatting or fetching related details.

Simply put:

• PRE-QUERY = Modify conditions before fetching data
• POST-QUERY = Take action after data is fetched

Example of PRE-QUERY Trigger

Here’s a practical example of how PRE-QUERY can be used:

BEGIN
   -- Restrict data based on the current user’s department
   IF :GLOBAL.DEPT_ID IS NOT NULL THEN
      :EMP.DEPT_ID := :GLOBAL.DEPT_ID;
   END IF;

   -- Add a default filter for active employees only
   :EMP.STATUS := 'ACTIVE';
END;

Explanation:

• The user’s department is taken from a global variable and automatically applied as a filter.
• The query is modified so that only employees with status ACTIVE are retrieved.
• The user does not need to manually enter these conditions.

Using PRE-QUERY with the ADDITIONAL WHERE Clause

Another advanced use of PRE-QUERY is dynamically appending WHERE clauses using the SET_BLOCK_PROPERTY built-in.

Example:

BEGIN
   -- Add a custom WHERE condition to restrict salary range
   SET_BLOCK_PROPERTY('EMP', DEFAULT_WHERE, 'SALARY > 3000 AND SALARY < 8000');
END;

Here, the default WHERE condition is set programmatically, and it will apply whenever a query is executed on the EMP block.

Best Practices for PRE-QUERY Trigger

To make the most of the PRE-QUERY Trigger, follow these best practices:

1. Keep conditions relevant
   • Do not overload the query with too many restrictions unless necessary.
2. Avoid hardcoding values
   • Use global variables, parameters, or user inputs for flexibility.
3. Use DEFAULT_WHERE carefully
   • Reset or clear it after usage if it should not persist across queries.
4. Test performance impact
   • Ensure additional WHERE clauses do not slow down queries unnecessarily.
5. Maintain security checks
   • Use PRE-QUERY to enforce user-specific restrictions that cannot be bypassed.

Common Mistakes to Avoid

While PRE-QUERY is powerful, developers often make mistakes that lead to unexpected results:

• Placing validation logic inside PRE-QUERY: Validation should occur in WHEN-VALIDATE-ITEM or similar triggers, not here.
• Overwriting user input: Ensure that automatic conditions do not erase what the user has already entered.
• Not resetting DEFAULT_WHERE: Leaving additional WHERE clauses active can cause confusion or missing records in future queries.
• Using it for post-processing: PRE-QUERY is not meant for formatting or calculations on fetched data; use POST-QUERY for that.

Benefits of Using PRE-QUERY Trigger

When used properly, the PRE-QUERY Trigger offers several benefits:

• Improved security: Restricts unauthorized access to data.
• User convenience: Automatically applies filters, saving time.
• Consistency: Ensures that certain conditions are always applied.
• Performance optimization: Reduces data load by fetching only what is necessary.

Conclusion

The PRE-QUERY Trigger in Oracle Forms is a vital tool for controlling data retrieval. By firing just before a query runs, it gives developers the power to add conditions, enforce security, and ensure users only see relevant data. When used alongside POST-QUERY, it helps create a well-structured data handling flow in Oracle Forms applications.

By following best practices and avoiding common mistakes, you can use the PRE-QUERY Trigger to make your forms more secure, efficient, and user-friendly. Whether you want to apply default filters, restrict access by department, or optimize performance, this trigger is one of the most effective ways to manage queries in Oracle Forms.