GROUP BY - do not group NULL - mysql

Question

I'm trying to figure out a way to return results by using the group by function.
GROUP BY is working as expected, but my question is: Is it possible to have a group by ignoring the NULL field. So that it does not group NULLs together because I still need all the rows where the specified field is NULL.
SELECT `table1`.*,
GROUP_CONCAT(id SEPARATOR ',') AS `children_ids`
FROM `table1`
WHERE (enabled = 1)
GROUP BY `ancestor`
So now let's say I have 5 rows and the ancestor field is NULL, it returns me 1 row....but I want all 5.

Answer 1

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

Answer 2

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.

Answer 3

GROUP BY IFNULL(required_field, id)

Answer 4

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

Answer 5

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`

Answer 6

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