User defined functions can be created with source code.

h2/src/test/org/h2/test/todo/createFunction.txt

@@ -13,3 +13,20 @@ VOLATILE value can change
 
 CALLED ON NULL INPUT
 RETURNS NULL ON NULL INPUT or STRICT
+
+CREATE FUNCTION add(integer, integer) RETURNS integer
+    AS 'select $1 + $2;'
+    LANGUAGE SQL
+    IMMUTABLE
+    RETURNS NULL ON NULL INPUT;
+    
+CREATE OR REPLACE FUNCTION increment(i integer) RETURNS integer AS $$
+        BEGIN
+                RETURN i + 1;
+        END;
+$$ LANGUAGE plpgsql;
+
+create alias newFunctionAliasName [ DETERMINISTIC ]
+as 
+FOR classAndMethodName
+