Refine api style for change set

script/sql/api/api.sql

@@ -0,0 +1,37 @@
+drop function if exists execute_command;
+drop function if exists execute_update;
+
+create function execute_update(cs jsonb) returns jsonb as
+$$
+declare
+begin
+    assert cs->>'operation' = 'update';
+
+    case cs->>'type'
+        when 'title' then
+            return update_title(cs);
+        else
+            return '{}'::jsonb;
+    end case;
+end;
+$$
+language plpgsql;
+
+
+create function execute_command(cs jsonb) returns jsonb as
+$$
+declare
+begin
+    case cs->>'operation'
+        when 'add' then
+            return '{}'::jsonb;
+        when 'update' then
+            return execute_update(cs);
+        when 'delete' then
+            return '{}'::jsonb;
+        else
+            return '{}'::jsonb;
+    end case;
+end;
+$$
+language plpgsql;