Unexcepted result in SELECT ... CASE WHEN ... ELSE NULL ...

[lovewin99]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table kankan1(id int, name text);

insert into kankan1 values(1, 'a');

insert into kankan1 values(2, 'a');

create table kankan2(id int, h1 text);

insert into kankan2 values(2, 'z');

select * from (
select
t1.id,
t2.h1,
	case 
	when t1.name='b' then 'case2'
	when t1.name='a' then 'case1'
	else null end as 'flag' 
from kankan1 t1
left join kankan2 t2 on t1.id = t2.id 
 ) t3 
where t3.flag = 'case1' order by t3.id;

2. What did you expect to see?

+------+------+-------+
| id   | h1   | flag  |
+------+------+-------+
|    1 | NULL | case1 |
|    2 | z    | case1 |
+------+------+-------+

3. What did you see instead?

+------+------+-------+
| id   | h1   | flag  |
+------+------+-------+
|    2 | z    | case1 |
+------+------+-------+

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Release Version: v3.0.0-rc.1-290-g21d2590ac
Git Commit Hash: 21d2590ac37bf53e5e687b8a341d50105ddbffe8
Git Branch: my-master
UTC Build Time: 2019-07-05 10:20:45
GoVersion: go version go1.12.5 darwin/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false

[zz-jason]

fixed by #11105