function group_concat ([order by ... desc...]), 'order by' and 'desc' do not work as same as mysql

[irene-coming]

• dble version：
  5.6.29-dble-9.9.9.9-08136d5-20181106070356

• preconditions ：
  no

• configs：
  schema.xml

<table name="aly_test" dataNode="dn1,dn2,dn3,dn4" rule="hash-four" />

• steps：
  step1. execute sqls with both dble and mysql:

CREATE TABLE `aly_test` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
insert into aly_test values(1,'a'),(null,'b'),(2,null),(1,'d'),(null,'c'),(3,'a'),(null,'b'),(null,'c'),(null,'b');

step2.execute sql with mysql

mysql> select *, group_concat(name order by name) from aly_test group by id;
+------+------+----------------------------------+
| id   | name | group_concat(name order by name) |
+------+------+----------------------------------+
| NULL | b    | b,b,b,c,c                        |
|    1 | a    | a,d                              |
|    2 | NULL | NULL                             |
|    3 | a    | a                                |
+------+------+----------------------------------+
4 rows in set (0.00 sec)

mysql> select *, group_concat(distinct name order by name desc separator ':') from aly_test group by id;
+------+------+--------------------------------------------------------------+
| id   | name | group_concat(distinct name order by name desc separator ':') |
+------+------+--------------------------------------------------------------+
| NULL | b    | c:b                                                          |
|    1 | a    | d:a                                                          |
|    2 | NULL | NULL                                                         |
|    3 | a    | a                                                            |
+------+------+--------------------------------------------------------------+
4 rows in set (0.01 sec)

step3. execute same sql as step2 with dble:

mysql> select *, group_concat(name order by name) from aly_test group by id;
+------+------+---------------------------------------------------------+
| id   | name | GROUP_CONCAT(aly_test.name ORDER BY name SEPARATOR ',') |
+------+------+---------------------------------------------------------+
| NULL | b    | b,c,b,c,b                                               |
|    1 | a    | a,d                                                     |
|    2 | NULL | NULL                                                    |
|    3 | a    | a                                                       |
+------+------+---------------------------------------------------------+
4 rows in set (0.01 sec)
mysql> select *, group_concat(distinct name order by name desc separator ':') from aly_test group by id;
+------+------+-----------------------------------------------------------------------+
| id   | name | GROUP_CONCAT(DISTINCT aly_test.name ORDER BY name DESC SEPARATOR ':') |
+------+------+-----------------------------------------------------------------------+
| NULL | b    | b:c                                                                   |
|    1 | a    | a:d                                                                   |
|    2 | NULL | NULL                                                                  |
|    3 | a    | a                                                                     |
+------+------+-----------------------------------------------------------------------+
4 rows in set (0.01 sec)

• expect result：
  1. step3 get same result as step2
• real result：
  1.
• supplements：
  1.autotest already append at version:674072355e4d2a503d15ce3234884f71ab0ad03a aggregate.sql, if issue fix, need update std_result

/label ~BUG

[FlyingMao]

for tester:
suggest autotest position: sqls/syntax/aggregate.sql(reference line:25、26)

[FlyingMao]

verified version: 5.6.29-dble-9.9.9.9-a68f557-20181119123440