Mysql With Rollup did not run what I expected - mysql

Question

Here is my example (mysql Ver 14.14 Distrib 5.7.21, for Win64 (x86_64))
DROP TABLE IF EXISTS t_tt;
CREATE TEMPORARY TABLE t_tt SELECT 1 AS tid,'team 1' AS teamName,111 AS teamData;
INSERT INTO t_tt VALUES(2,'team 2',222);
INSERT INTO t_tt VALUES(3,'team 3',333);
SELECT
tid,
isnull(tid),
IF(isnull(tid),'total',teamName) AS displayName,
SUM(teamData)
FROM t_tt GROUP BY tid WITH ROLLUP;
And I think the result must be ：
+-----+-------------+-------------+---------------+
| tid | isnull(tid) | displayName | SUM(teamData) |
+-----+-------------+-------------+---------------+
| 1 | 0 | team 1 | 111 |
| 2 | 0 | team 2 | 222 |
| 3 | 0 | team 3 | 333 |
| 3 | 1 | total | 666 |
+-----+-------------+-------------+---------------+
Actually，the real answer is ：
+-----+-------------+-------------+---------------+
| tid | isnull(tid) | displayName | SUM(teamData) |
+-----+-------------+-------------+---------------+
| 1 | 0 | team 2 | 111 |
| 2 | 0 | team 3 | 222 |
| 3 | 0 | team 3 | 333 |
| NULL| 0 | team 3 | 666 |
+-----+-------------+-------------+---------------+
I dont know why the column of displayName is begins with "team 2" ,but not "team 1".
And the last row isnull(tid) should equals 1 , but not 0.

Answer 1

The problem is that you are using a MySQL extension. You have teamName in the SELECT, but it is not in the GROUP BY. So, MySQL is free to take this value from any row. To be honest, I thought it would need to be from a matching row. I don't fully understand the behavior.
But, this is easy to fix. One simple method is simply to add an aggregation function:
SELECT tid, tid is null,
COALESCE(max(teamName), 'total') AS displayName,
SUM(teamData)
FROM t_tt
GROUP BY tid WITH ROLLUP;
Note that for the additional row from the ROLLUP, the value is arbitrary.
I also found that the results were fixed if the table were pre-declared with a primary key:
CREATE TABLE t_tt (
tid int primary key,
teamName varchar(255),
teamData int
);
INSERT INTO t_tt VALUES(1, 'team 1', 111);
INSERT INTO t_tt VALUES(2, 'team 2', 222);
INSERT INTO t_tt VALUES(3, 'team 3', 333);

Answer 2

It's better to process all the extra information out of the group query, since some columns not grouped could have an arbitrary value:
SELECT tid,
isnull(tid),
IF(isnull(tid),'total',teamName) AS displayName,
team_sum
FROM
(
SELECT
tid,
teamName,
SUM(teamData) as team_sum
FROM t_tt
GROUP BY tid WITH ROLLUP
) t
See this Sql Fiddle: http://sqlfiddle.com/#!9/8871cc/9