Empowering Your SQL Queries with User-Defined Functions in System.Data.SQLite

System.Data.SQLite allows you to extend its built-in functionalities by defining custom UDFs. These functions can perform various tasks, such as string manipulation, date calculations, or complex aggregations, enriching your SQL queries.

Here's a step-by-step guide on creating UDFs:

1. Define the Function:

   • Create a static method in your C# code that takes the desired parameters and returns a value.
   • Use appropriate data types for parameters and the return value. For example:

   public static string MyUpperCaseFunction(string input)
   {
       return input.ToUpper();
   }
   

2. • Open a connection to your SQLite database using SQLiteConnection.
   • Call the CreateFunction method on the connection, passing the function name, number of arguments, and a delegate representing the function implementation:

   using (var connection = new SQLiteConnection("your_database.db"))
   {
       connection.CreateFunction("MyUpperCase", MyUpperCaseFunction);
   }
   

Using UDFs in SQL Queries

Once registered, you can directly invoke your UDFs within your SQL queries:

SELECT MyUpperCase("hello world") AS upper_text;

This query will call the MyUpperCase function and display the uppercase version of "hello world".

Example: Custom String Length Function

Here's a more concrete example of creating a UDF to calculate the length of a string, excluding leading and trailing whitespaces:

public static int CustomStringLength(string input)
{
    return input.Trim().Length;
}

using (var connection = new SQLiteConnection("your_database.db"))
{
    connection.CreateFunction("CustomStringLength", CustomStringLength);
}

// Usage in SQL query:
SELECT CustomStringLength("  trimmed string  ") AS custom_length;

Important Considerations:

• Ensure the function name is unique within the database.
• For performance optimization, mark deterministic functions using the isDeterministic argument in CreateFunction. This allows SQLite to potentially reuse pre-computed results.