How to Call a Function via Trigger in Oracle SQL

Discover how to effectively use triggers and procedures in Oracle SQL to log changes to your database—specifically, learn how to call a function through a trigger and handle data modifications seamlessly.
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How to Call a Function via Trigger in Oracle SQL

Managing data in a database requires not just efficient handling of data but also ensuring that historical changes are recorded properly. In Oracle SQL, this can often be achieved through the use of triggers and functions. If you're working with Oracle Database XE 18c and SQL Developer, you may find yourself in a situation where you need to call a function through a trigger when a specific event occurs. Let’s break down how this can be accomplished in an engaging and informative manner.

The Case at Hand

In this instance, you have two tables: one named courses and the other course_history. The courses table holds current course information while the course_history table is meant to store historical data, like previous credit values when changes occur. You want to create a mechanism that captures changes to the credit attribute in the courses table, maintaining a record of past values automatically whenever an update is made.

Problem Breakdown

The question arises: How can you effectively call a function inside a trigger? The initial code provided largely suggests using a function to perform this operation. However, the recommended approach in Oracle SQL is to use procedures instead.

Why Use a Procedure Instead of a Function?

Functions are designed primarily for reading data and returning a value. They shouldn't be used for tasks that modify data.

Procedures are suited for data manipulation tasks because they can modify database state without returning a value.

The Solution: Implementing a Procedure

Let’s dive into how you can correctly structure your SQL code for this scenario, utilizing a procedure in place of a function.

Step 1: Create the Procedure

First, you need to set up a procedure that takes in the old values from the courses table and inserts them into the course_history table. Here’s a revised example of your SQL code:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Create the Trigger

Next, implement a trigger that calls this procedure whenever there’s an update on the credit attribute in the courses table. Here is how you can do that:

[[See Video to Reveal this Text or Code Snippet]]

Important Notes

Usage of Pseudorecords: In the context of triggers, :OLD and :NEW pseudorecords are used to reference the values before and after an update, respectively. In this case, we used :OLD to capture previous values to log into the history table.

No Return for Procedures: Unlike functions, the procedure does not need to return a value as its primary role is to manipulate data in the database.

Final Thoughts

By following the above instructions, you can efficiently log changes in your database, ensuring that historical data is preserved with each update to the courses table. The structured use of procedures and triggers will contribute to maintaining data integrity within your Oracle SQL databases.

If you have any questions or need further assistance, feel free to reach out! Happy coding!