Fix #105 - Changed G-4220 examples to use constants instead of literals

Author: kibeha

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

@@ -11,52 +11,52 @@
 
 ``` sql
 -- @formatter:off
-select decode(dummy, 'X', 1 
-                   , 'Y', 2
-                   , 'Z', 3
-                        , 0)
-  from dual;
+select decode(ctry.country_code, constants_up.co_ctry_uk, constants_up.co_lang_english 
+                               , constants_up.co_ctry_fr, constants_up.co_lang_french
+                               , constants_up.co_ctry_de, constants_up.co_lang_german
+                                                        , constants_up.co_lang_not_supported)
+  from countries ctry;
 ```
 
-## Example (good)
+`null` values can be compared in `decode`:
 
 ``` sql
-select case dummy
-          when 'X' then
-             1
-          when 'Y' then
-             2
-          when 'Z' then
-             3
-          else
-             0
-       end
-  from dual;
+-- @formatter:off
+select decode(ctry.country_code, constants_up.co_ctry_uk, constants_up.co_lang_english 
+                               , constants_up.co_ctry_fr, constants_up.co_lang_french
+                               , null                   , constants_up.co_lang_unknown
+                                                        , constants_up.co_lang_not_supported)
+  from countries ctry;
 ```
 
-## Example (bad)
+## Example (good)
 
 ``` sql
--- @formatter:off
-select decode(dummy,  'X',  1 
-                   ,  'Y',  2
-                   , null, -1
-                         ,  0)
-  from dual;
+select case ctry.country_code
+          when constants_up.co_ctry_uk then
+             constants_up.co_lang_english
+          when constants_up.co_ctry_fr then
+             constants_up.co_lang_french
+          when constants_up.co_ctry_de then
+             constants_up.co_lang_german
+          else
+             constants_up.co_lang_not_supported
+       end
+  from countries ctry;
 ```
 
-## Example (good)
+Simple `case` can not compare `null` values, instead the searched `case` expression must be used:
 
 ``` sql
 select case
-          when dummy = 'X' then
-             1
-          when dummy = 'Y' then
-             2
-          when dummy is null then
-             -1
+          when ctry.country_code = constants_up.co_ctry_uk then
+             constants_up.co_lang_english
+          when ctry.country_code = constants_up.co_ctry_fr then
+             constants_up.co_lang_french
+          when ctry.country_code is null then
+             constants_up.co_lang_unknown
           else
-             0
+             constants_up.co_lang_not_supported
        end
-  from dual;
+  from countries ctry;
 ```