How to Create a User-Defined Function in SQLite

How to create custom functions in SQLite

SQLite does not have a stored function/stored procedure language. So CREATE FUNCTION does not work. What you can do though is map functions from a c library to SQL functions (user-defined functions). To do that, use SQLite's C API (see: http://www.sqlite.org/c3ref/create_function.html)

If you're not using the C API, your wrapper API may define something that allows you access to this feature, see for example:

• PHP sqlite_create_function() (http://www.php.net/manual/en/function.sqlite-create-function.php)
• Python sqlite3.create_function() (http://docs.python.org/3/library/sqlite3.html#sqlite3.Connection.create_function)
• Perl $dbh->sqlite_create_function($name,$argc,$code_ref,$flags) (https://metacpan.org/pod/DBD::SQLite#$dbh-%3Esqlite_create_function(-$name,-$argc,-$code_ref,-$flags-))

How can I create a user-defined function in SQLite?

SQLite does not have support for user-defined functions in the way that Oracle or MS SQL Server does. For SQLite, you must create a callback function in C/C++ and hook the function up using the sqlite3_create_function call.

Unfortunately, the SQLite API for Android does not allow for the sqlite3_create_function call directly through Java. In order to get it to work you will need to compile the SQLite C library with the NDK.

And if you are still interested read 2.3 User-defined functions...

Here's how to create a function that finds the first byte of a string.

static void firstchar(sqlite3_context *context, int argc, sqlite3_value **argv)
{
    if (argc == 1) {
        char *text = sqlite3_value_text(argv[0]);
        if (text && text[0]) {
          char result[2]; 
          result[0] = text[0]; result[1] = '\0';
          sqlite3_result_text(context, result, -1, SQLITE_TRANSIENT);
          return;
        }
    }
    sqlite3_result_null(context);
}

Then attach the function to the database.

sqlite3_create_function(db, "firstchar", 1, SQLITE_UTF8, NULL, &firstchar, NULL, NULL)

Finally, use the function in a sql statement.

SELECT firstchar(textfield) from table

Is it possible to create a javascript User-defined function in sqlite

Yes. It is possible to call javascript functions

//(thanks to Mirnal Kant, SQLManager)
//Version 2 -- Prevent Firefox crashing 
//          -- Suspect a problem with continual creation of Regex objects

var g_RegExpString = null;
var g_RegExp = null;

//functions to be created for the db
var smDbFunctions = { 
  // (0) = Regex Expression
  // (1) = Column value to test
    regexp: {
        onFunctionCall: function(val) {
            if (g_RegExp == null || val.getString(0) != g_RegExpString) 
            {
                g_RegExpString = val.getString(0);
                g_RegExp = new RegExp(g_RegExpString);
            }
            if (val.getString(1).match(g_RegExp)) return 1;
            else return 0;
        }
    }
}; 

after instantiating a SQLite instance:

Database.createFunction("REGEXP", 2, smDbFunctions.regexp);

How do I call user-defined function in SQLite only if it exists?

SQLite is designed as an embedded database, so it is assumed that your application controls what is done with the database.

SQLite has no SQL function to check for user-defined functions.

If you want to detect changes made only from your own program, use sqlite3_update_hook.

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