Fix for bug #17896: MIN of CASE WHEN returns non-minimum value!

sql/item_cmpfunc.cc:
  Fix for bug #17896: MIN of CASE WHEN returns non-minimum value!
  - NULL items should not affect the result type.

mysql-test/r/case.result

@@ -169,3 +169,11 @@ SELECT CASE '1' WHEN '2' THEN 'BUG' ELSE 'nobug' END;
 case+union+test
 case+union+test
 nobug
+create table t1(a float, b int default 3);
+insert into t1 (a) values (2), (11), (8);
+select min(a), min(case when 1=1 then a else NULL end),
+min(case when 1!=1 then NULL else a end) 
+from t1 where b=3 group by b;
+min(a)	min(case when 1=1 then a else NULL end)	min(case when 1!=1 then NULL else a end)
+2	2	2
+drop table t1;

mysql-test/t/case.test

@@ -119,4 +119,15 @@ SELECT 'case+union+test'
 UNION 
 SELECT CASE '1' WHEN '2' THEN 'BUG' ELSE 'nobug' END;
 
+#
+# Bug #17896: problem with MIN(CASE...)
+#
+
+create table t1(a float, b int default 3);
+insert into t1 (a) values (2), (11), (8);
+select min(a), min(case when 1=1 then a else NULL end),
+  min(case when 1!=1 then NULL else a end) 
+from t1 where b=3 group by b;
+drop table t1;
+
 # End of 4.1 tests

sql/item_cmpfunc.cc

@@ -37,10 +37,21 @@ static Item_result item_store_type(Item_result a,Item_result b)
 
 static void agg_result_type(Item_result *type, Item **items, uint nitems)
 {
-  uint i;
-  type[0]= items[0]->result_type();
-  for (i=1 ; i < nitems ; i++)
-    type[0]= item_store_type(type[0], items[i]->result_type());
+  Item **item, **item_end;
+
+  /* Note: NULL items don't affect the result type */
+  *type= STRING_RESULT;
+  /* Skip beginning NULL items */
+  for (item= items, item_end= item + nitems; item < item_end; item++)
+    if ((*item)->type() != Item::NULL_ITEM)
+    {
+      *type= (*item)->result_type();
+      item++;
+      break;
+    }
+  for (; item < item_end; item++)
+    if ((*item)->type() != Item::NULL_ITEM)
+      *type= item_store_type(type[0], (*item)->result_type());
 }
 
 static void agg_cmp_type(Item_result *type, Item **items, uint nitems)