Add CASE expression documentation

Author: arnaud-lacurie

docs/sphinx/source/reference/sql_commands/DQL.rst

@@ -21,6 +21,7 @@ Syntax
     DQL/SELECT
     DQL/WITH
     DQL/WHERE
+    DQL/CASE
     DQL/EXPLAIN
 
 .. toctree::

docs/sphinx/source/reference/sql_commands/DQL/CASE.diagram

@@ -0,0 +1,19 @@
+Diagram(
+    Terminal('CASE'),
+    Optional(NonTerminal('expression')),
+    OneOrMore(
+        Sequence(
+            Terminal('WHEN'),
+            NonTerminal('condition'),
+            Terminal('THEN'),
+            NonTerminal('result')
+        )
+    ),
+    Optional(
+        Sequence(
+            Terminal('ELSE'),
+            NonTerminal('result')
+        )
+    ),
+    Terminal('END')
+)