Troubleshooting Oracle SQL: How to Fix User Defined Functions Not Running in SELECT Statements

Discover solutions for why your Oracle SQL user-defined function does not run in a SELECT statement while working inside a procedure. Learn how to modify your function return type for successful execution.
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This video is based on the question https://stackoverflow.com/q/77263853/ asked by the user 'Chriscolle' ( https://stackoverflow.com/u/15255536/ ) and on the answer https://stackoverflow.com/a/77263891/ provided by the user 'Koen Lostrie' ( https://stackoverflow.com/u/4189814/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting Oracle SQL: User Defined Functions and SELECT Statements

When working with Oracle SQL, many users encounter various challenges creating and executing functions. One common issue arises when attempting to run a user-defined function in a SELECT statement, only to find that it fails to execute properly—even though it works flawlessly when wrapped in a procedure. In this post, we'll explore the problem, its underlying causes, and the solution that will get your function running correctly in a SELECT statement.

Understanding the Problem

The Function

You've created a function named PreviouslyVoted, designed to determine whether a voter has cast a ballot in a specific election. Here is the essence of your function:

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The Error

When executing a query like this:

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You encounter the following error message:

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This is frustrating, especially since your function works perfectly fine when invoked within a procedure:

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What’s Going Wrong?

The root cause of your issue lies in the return type of your function. The previouslyVoted function returns a BOOLEAN type, which is supported in PL/SQL but not recognized in SQL until Oracle version 23c. Most SQL statements—including your SELECT statement—do not understand how to handle BOOLEAN values and will throw the invalid datatype error.

A Simple Explanation

PL/SQL allows the use of BOOLEAN as a data type.

Oracle SQL does not support BOOLEAN return types before version 23c.

When you try to use your function in a SELECT statement, it doesn't know how to interpret the output of type BOOLEAN, leading to the error.

The Solution: Modify the Return Type

To resolve this issue, you need to change the function's return type to a data type that Oracle SQL recognizes, such as VARCHAR2 or NUMBER. This allows you to signal success or failure in a way that SQL can comprehend.

Revised Function Example

You can modify your function to return a string indicating whether the voter has previously voted:

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The SELECT Statement

You can now easily run the query without errors:

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Conclusion

By adjusting the return type of your user-defined function from BOOLEAN to VARCHAR2, you ensure compatibility with SQL SELECT statements while still preserving the functionality of your original intent. Always remember this key difference between PL/SQL and SQL datatype handling when working with functions in Oracle.

If you encounter any further questions or challenges, feel free to reach out or consult the Oracle documentation for additional guidance!