Fix #85 - Warn about NULL issues in rule G-4220

Author: kibeha

docs/4-language-usage/4-control-structures/2-case-if-decode-nvl-nvl2-coalesce/g-4220.md

@@ -5,7 +5,7 @@
 
 ## Reason
 
-`DECODE` is an ORACLE specific function hard to understand and restricted to SQL only. The “newer” `CASE` function is much more common has a better readability and may be used within PL/SQL too.
+`decode` is an ORACLE specific function hard to understand and restricted to SQL only. The “newer” `case` function is much more common, has a better readability and may be used within PL/SQL too. Be careful that `decode` can handle `null` values, which the simple `case` cannot - for such cases you must use the searched `case` and `is null` instead.
 
 ## Example (bad)
 
@@ -27,4 +27,26 @@ select case dummy
           else 0
        end
   from dual;
+```
+
+## Example (bad)
+
+``` sql
+select decode(dummy,  'X',  1 
+                   ,  'Y',  2
+                   , null, -1
+                         ,  0)
+  from dual;
+```
+
+## Example (good)
+
+``` sql
+select case
+          when dummy = 'X'   then  1
+          when dummy = 'Y'   then  2
+          when dummy is null then -1
+          else 0
+       end
+  from dual;
 ```