MariaDBでGROUP BY句WITH ROLLUPを使用した2つの同様のクエリの作業の相違

Question

次の2つのクエリが異なる結果を返す理由を理解してください。クエリの違いは、GROUP BY句でDATE_FORMAT関数を使用する場合のみです。

クエリ1：

SELECT DATE_FORMAT(T0.ET,'%Y/%m/%d %H%i'), SUM(T0.AT) 
FROM(SELECT TIMESTAMPADD(second,(SEQ -1)*300, STR_TO_DATE('20170809135000','%Y%m%d%H%i%s')) ET, 1 AT 
    FROM SEQ_1_TO_10 
    WHERE SEQ <= 5) T0 
    group by T0.ET with rollup 

結果1：

'2017/08/09 1350', '1' 
'2017/08/09 1355', '1' 
'2017/08/09 1400', '1' 
'2017/08/09 1405', '1' 
'2017/08/09 1410', '1' 
NULL, '5' 

クエリ2：

SELECT DATE_FORMAT(T0.ET,'%Y/%m/%d %H%i') , SUM(T0.AT) 
FROM(SELECT TIMESTAMPADD(second,(SEQ -1)*300, STR_TO_DATE('20170809135000','%Y%m%d%H%i%s')) ET, 1 AT 
    FROM SEQ_1_TO_10 
    WHERE SEQ <= 5) T0 
    group by DATE_FORMAT(T0.ET,'%Y/%m/%d %H%i') with rollup 

結果2：

'2017/08/09 1350', '1' 
'2017/08/09 1355', '1' 
'2017/08/09 1400', '1' 
'2017/08/09 1405', '1' 
'2017/08/09 1410', '1' 
'2017/08/09 1410', '5' 

は、両方のクエリのSELECT EXPLAIN同じです：

id, select_type, table,   type, possible_keys, key,  key_len, ref, rows, Extra 
'1', 'SIMPLE', 'SEQ_1_TO_10', 'range', 'PRIMARY',  'PRIMARY','8',  NULL, '5', 'Using where; Using index; Using filesort' 

Answer 1

Welcome to the MariaDB monitor. Commands end with ; or \g. 
Your MariaDB connection id is 13 
Server version: 10.1.14-MariaDB mariadb.org binary distribution 

Copyright (c) 2000, 2016, Oracle, MariaDB Corporation Ab and others. 

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. 

MariaDB [sandbox]> SELECT typ ,DATE_FORMAT(T.ET,'%Y/%m/%d %H%i') et, sum(1) 
    -> FROM(
    -> SELECT TIMESTAMPADD(second,(SEQ -1)*300, STR_TO_DATE('20170809135000','%Y%m%d%H%i%s')) ET, 
    -> case when seq % 2 = 0 then 'a' else 'b' end as typ 
    -> FROM SEQ_1_TO_10 
    -> WHERE SEQ <= 5 
    ->) T 
    -> group by typ,DATE_FORMAT(T.ET,'%Y/%m/%d %H%i') with rollup; 
+------+-----------------+--------+ 
| typ | et    | sum(1) | 
+------+-----------------+--------+ 
| a | 2017/08/09 1355 |  1 | 
| a | 2017/08/09 1405 |  1 | 
| a | 2017/08/09 1405 |  2 | 
| b | 2017/08/09 1350 |  1 | 
| b | 2017/08/09 1400 |  1 | 
| b | 2017/08/09 1410 |  1 | 
| b | 2017/08/09 1410 |  3 | 
| NULL | 2017/08/09 1410 |  5 | 
+------+-----------------+--------+ 
8 rows in set (0.00 sec)