Fixed bug #49600.

The problem could be demonstrated with an outer join of two single-row
tables where the values of the join attributes were null. Any query
with such a join could return a wrong result set if the where
condition of the query was not empty. For queries with empty
where conditions the result sets were correct.
This was the consequence of two bugs in the code:
 - Item_equal objects for on conditions of outer joins were
   not built if the processed query had no where condition
 - the check for null values in the code that evaluated constant 
   Item_equal objects was incorrect.
Fixed both above problems.
Added a test case for the bug and adjusted results for some other
test cases.

mysql-test/r/join_outer.result

@@ -1397,4 +1397,18 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 Warnings:
 Note	1003	select straight_join `test`.`jt1`.`f1` AS `f1` from `test`.`t1` `jt6` left join (`test`.`t1` `jt3` join `test`.`t1` `jt4` left join `test`.`t1` `jt5` on(1) left join `test`.`t1` `jt2` on(1)) on((`test`.`jt6`.`f1` and 1)) left join `test`.`t1` `jt1` on(1) where 1
 DROP TABLE t1;
+#
+# Bug#49600: outer join of two single-row tables with joining attributes
+#            evaluated to nulls
+create table t1 (a int, b int);
+create table t2 (a int, b int);
+insert into t1 values (1, NULL);
+insert into t2 values (2, NULL);
+select * from t1 left join t2 on t1.b=t2.b;
+a	b	a	b
+1	NULL	NULL	NULL
+select * from t1 left join t2 on t1.b=t2.b where 1=1;
+a	b	a	b
+1	NULL	NULL	NULL
+drop table t1,t2;
 End of 5.1 tests

mysql-test/r/table_elim.result

@@ -60,7 +60,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t0	ALL	NULL	NULL	NULL	NULL	4	100.00	
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	100.00	
 Warnings:
-Note	1003	select `test`.`t0`.`a` AS `a` from `test`.`t0` left join (`test`.`t1`) on((`test`.`t0`.`a` = `test`.`t1`.`a`)) where 1
+Note	1003	select `test`.`t0`.`a` AS `a` from `test`.`t0` left join (`test`.`t1`) on((`test`.`t1`.`a` = `test`.`t0`.`a`)) where 1
 # Elimination with aggregate functions
 explain select count(*) from t1 left join t2 on t2.a=t1.a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

mysql-test/r/view.result

@@ -1418,7 +1418,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	100.00	
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	
 Warnings:
-Note	1003	select `test`.`t3`.`a` AS `a`,`test`.`t1`.`a` AS `a`,`test`.`t2`.`a` AS `b` from `test`.`t3` left join (`test`.`t1` left join `test`.`t2` on((`test`.`t1`.`a` = `test`.`t2`.`a`))) on((`test`.`t3`.`a` = `test`.`t1`.`a`)) where 1
+Note	1003	select `test`.`t3`.`a` AS `a`,`test`.`t1`.`a` AS `a`,`test`.`t2`.`a` AS `b` from `test`.`t3` left join (`test`.`t1` left join `test`.`t2` on(((`test`.`t1`.`a` = `test`.`t3`.`a`) and (`test`.`t2`.`a` = `test`.`t3`.`a`)))) on((`test`.`t1`.`a` = `test`.`t3`.`a`)) where 1
 create view v1 (a) as select a from t1;
 create view v2 (a) as select a from t2;
 create view v4 (a,b) as select v1.a as a, v2.a as b from v1 left join v2 on (v1.a=v2.a);
@@ -1433,7 +1433,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	100.00	
 1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	2	100.00	
 Warnings:
-Note	1003	select `test`.`t3`.`a` AS `a`,`test`.`t1`.`a` AS `a`,`test`.`t2`.`a` AS `b` from `test`.`t3` left join (`test`.`t1` left join (`test`.`t2`) on((`test`.`t1`.`a` = `test`.`t2`.`a`))) on((`test`.`t3`.`a` = `test`.`t1`.`a`)) where 1
+Note	1003	select `test`.`t3`.`a` AS `a`,`test`.`t1`.`a` AS `a`,`test`.`t2`.`a` AS `b` from `test`.`t3` left join (`test`.`t1` left join (`test`.`t2`) on(((`test`.`t1`.`a` = `test`.`t3`.`a`) and (`test`.`t2`.`a` = `test`.`t3`.`a`)))) on((`test`.`t1`.`a` = `test`.`t3`.`a`)) where 1
 prepare stmt1 from "select * from t3 left join v4 on (t3.a = v4.a);";
 execute stmt1;
 a	a	b

mysql-test/t/join_outer.test

@@ -981,4 +981,19 @@ EXPLAIN EXTENDED SELECT STRAIGHT_JOIN jt1.f1 FROM t1 AS jt1
 
 DROP TABLE t1;
 
+--echo #
+--echo # Bug#49600: outer join of two single-row tables with joining attributes
+--echo #            evaluated to nulls
+
+create table t1 (a int, b int);
+create table t2 (a int, b int);
+insert into t1 values (1, NULL);
+insert into t2 values (2, NULL);
+
+select * from t1 left join t2 on t1.b=t2.b;
+
+select * from t1 left join t2 on t1.b=t2.b where 1=1;
+
+drop table t1,t2;
+
 --echo End of 5.1 tests

sql/item_cmpfunc.cc

@@ -5507,7 +5507,7 @@ longlong Item_equal::val_int()
     return 0;
   List_iterator_fast<Item_field> it(fields);
   Item *item= const_item ? const_item : it++;
-  if ((null_value= item->null_value))
+  if ((null_value= item->is_null()))
     return 0;
   eval_item->store_value(item);
   while ((item_field= it++))
@@ -5515,7 +5515,7 @@ longlong Item_equal::val_int()
     /* 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))
+      if ((null_value= item_field->is_null()) || eval_item->cmp(item_field))
         return 0;
     }
   }

sql/sql_select.cc

@@ -9315,7 +9315,10 @@ optimize_cond(JOIN *join, COND *conds, List<TABLE_LIST> *join_list,
   DBUG_ENTER("optimize_cond");
 
   if (!conds)
+  {
     *cond_value= Item::COND_TRUE;
+    build_equal_items(join->thd, NULL, NULL, join_list, &join->cond_equal);
+  }  
   else
   {
     /*