组 BY-不要组 NULL

Perhaps you should add something to the null columns to make them unique and group on that? I was looking for some sort of sequence to use instead of UUID() but this might work just as well.

SELECT `table1`.*,
IFNULL(ancestor,UUID()) as unq_ancestor
GROUP_CONCAT(id SEPARATOR ',') AS `children_ids`
FROM `table1`
WHERE (enabled = 1)
GROUP BY unq_ancestor

Maybe faster version of previous solution in case you have unique identifier in table1 (let suppose it is table1.id) :

SELECT `table1`.*,
GROUP_CONCAT(id SEPARATOR ',') AS `children_ids`,
IF(ISNULL(ancestor),table1.id,NULL) as `do_not_group_on_null_ancestor`
FROM `table1`
WHERE (enabled = 1)
GROUP BY `ancestor`, `do_not_group_on_null_ancestor`

SELECT table1.*,
GROUP_CONCAT(id SEPARATOR ',') AS children_ids
FROM table1
WHERE (enabled = 1)
GROUP BY ancestor
, CASE WHEN ancestor IS NULL
THEN table1.id
ELSE 0
END

When grouping by column Y, all rows for which the value in Y is NULL are grouped together.

This behaviour is defined by the SQL-2003 standard, though it's slightly surprising because NULL is not equal to NULL.

You can work around it by grouping on a different value, some function (mathematically speaking) of the data in your grouping column.

If you have a unique column X then this is easy.

Input

X      Y
-------------
1      a
2      a
3      b
4      b
5      c
6      (NULL)
7      (NULL)
8      d

Without fix

SELECT GROUP_CONCAT(`X`)
FROM `tbl`
GROUP BY `Y`;

Result:

GROUP_CONCAT(`foo`)
-------------------
6,7
1,2
3,4
5
8

With fix

SELECT GROUP_CONCAT(`X`)
FROM `tbl`
GROUP BY IFNULL(`Y`, `X`);

Result:

GROUP_CONCAT(`foo`)
-------------------
6
7
1,2
3,4
5
8

Let's take a closer look at how this is working

SELECT GROUP_CONCAT(`X`), IFNULL(`Y`, `X`) AS `grp`
FROM `tbl`
GROUP BY `grp`;

Result:

GROUP_CONCAT(`foo`)     `grp`
-----------------------------
6                       6
7                       7
1,2                     a
3,4                     b
5                       c
8                       d

If you don't have a unique column that you can use, you can try to generate a unique placeholder value instead. I'll leave this as an exercise to the reader.

GROUP BY IFNULL(required_field, id)

To union multiple tables and group_concat different column and a sum of the column for the (unique primary or foreign key) column to display a value in the same row

select column1,column2,column3,GROUP_CONCAT(if(column4='', null, column4)) as
column4,sum(column5) as column5
from (
select column1,group_concat(column2) as column2,sum(column3 ) as column3,'' as
column4,'' as column5
from table1
group by column1


union all


select column1,'' as column2,'' as column3,group_concat(column4) as
column4,sum(column5) as column5
from table 2
group by column1
) as t
group by column1