diff --git a/mysql-test/r/func_group.result b/mysql-test/r/func_group.result
index 2b0176179ed..cfa4cc0ef68 100644
--- a/mysql-test/r/func_group.result
+++ b/mysql-test/r/func_group.result
@@ -865,6 +865,7 @@ select 1, min(a) from t1m where 1=99;
 1	NULL
 select 1, min(1) from t1m where a=99;
 1	min(1)
+1	NULL
 select 1, min(1) from t1m where 1=99;
 1	min(1)
 1	NULL
@@ -876,6 +877,7 @@ select 1, max(a) from t1m where 1=99;
 1	NULL
 select 1, max(1) from t1m where a=99;
 1	max(1)
+1	NULL
 select 1, max(1) from t1m where 1=99;
 1	max(1)
 1	NULL
diff --git a/mysql-test/r/select.result b/mysql-test/r/select.result
index 598ea2b10d1..198493d0a02 100644
--- a/mysql-test/r/select.result
+++ b/mysql-test/r/select.result
@@ -3337,3 +3337,16 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	const	PRIMARY	PRIMARY	4	const	1	Using index
 1	SIMPLE	t3	const	PRIMARY	PRIMARY	8	const,const	1	
 DROP TABLE t1,t2,t3;
+create table t1 (f1 int unique);
+create table t2 (f2 int unique);
+create table t3 (f3 int unique);
+insert into t1 values(1),(2);
+insert into t2 values(1),(2);
+insert into t3 values(1),(NULL);
+select * from t3 where f3 is null;
+f3
+NULL
+select t2.f2 from t1 left join t2 on f1=f2 join t3 on f1=f3 where f1=1;
+f2
+1
+drop table t1,t2,t3;
diff --git a/mysql-test/t/select.test b/mysql-test/t/select.test
index a73d08f5f18..595362dbb65 100644
--- a/mysql-test/t/select.test
+++ b/mysql-test/t/select.test
@@ -2805,3 +2805,17 @@ EXPLAIN SELECT t2.key_a,foo
     WHERE t2.key_a=2 and key_b=5;
 
 DROP TABLE t1,t2,t3;
+
+#
+# Bug #15633 Evaluation of Item_equal for non-const table caused wrong
+#            select result
+#
+create table t1 (f1 int unique);
+create table t2 (f2 int unique);
+create table t3 (f3 int unique);
+insert into t1 values(1),(2);
+insert into t2 values(1),(2);
+insert into t3 values(1),(NULL);
+select * from t3 where f3 is null;
+select t2.f2 from t1 left join t2 on f1=f2 join t3 on f1=f3 where f1=1;
+drop table t1,t2,t3;
diff --git a/sql/item_cmpfunc.cc b/sql/item_cmpfunc.cc
index 15614a32c39..306277aa438 100644
--- a/sql/item_cmpfunc.cc
+++ b/sql/item_cmpfunc.cc
@@ -3765,6 +3765,7 @@ void Item_equal::update_used_tables()
 
 longlong Item_equal::val_int()
 {
+  Item_field *item_field;
   if (cond_false)
     return 0;
   List_iterator_fast<Item_field> it(fields);
@@ -3772,10 +3773,14 @@ longlong Item_equal::val_int()
   if ((null_value= item->null_value))
     return 0;
   eval_item->store_value(item);
-  while ((item= it++))
+  while ((item_field= it++))
   {
-    if ((null_value= item->null_value) || eval_item->cmp(item))
-      return 0;
+    /* Skip fields of non-const tables. They haven't been read yet */
+    if (item_field->field->table->const_table)
+    {
+      if ((null_value= item_field->null_value) || eval_item->cmp(item_field))
+        return 0;
+    }
   }
   return 1;
 }
diff --git a/sql/item_cmpfunc.h b/sql/item_cmpfunc.h
index bfd32223d4c..89d117ee4ec 100644
--- a/sql/item_cmpfunc.h
+++ b/sql/item_cmpfunc.h
@@ -1150,6 +1150,11 @@ public:
   are deleted in the end of execution. All changes made to these
   objects need not be registered in the list of changes of the parse
   tree and do not harm PS/SP re-execution.
+
+  Item equal objects are employed only at the optimize phase. Usually they are
+  not supposed to be evaluated.  Yet in some cases we call the method val_int()
+  for them. We have to take care of restricting the predicate such an
+  object represents f1=f2= ...=fn to the projection of known fields fi1=...=fik.
 */
 
 class Item_equal: public Item_bool_func