How to GROUP_CONCAT where there are null values in MYSQL?

问题

I have a long query that returns multiple values for a primary key (from a LEFT join). For example : (only showing two fields, but there about 10 fields)

LotID    Size
1         A
1         B 
1         C
2         null
3         B
4         A
4         B

When I use GROUP_CONACT, it returns as follows :

LotID       Size
1           A,B,C
3           B
4           A,B   

But what I actually want is :

LotID       Size
1           A,B,C
2           null
3           B
4           A,B   

I tried using

GROUP_CONCAT(CONCAT_WS(',', IFNULL(Size,''))) AS Sizes,

It returns :

    LotID       Sizes
    1           A,B,C,,,
    3           B,,
    4           A,B,,  

It does not return LotID=2, also aditional commas.

How could I do it to get clean records ?

回答1:

You must be doing something wrong with group_concat, because this:

select 
  lotid,
  group_concat(size) size
from tablename
group by lotid

returns:

| lotid | size               |
| ----- | ------------------ |
| 1     | A,B,C              |
| 2     | null               |
| 3     | B                  |
| 4     | A,B                |

See the demo.

来源：https://stackoverflow.com/questions/55442539/how-to-group-concat-where-there-are-null-values-in-mysql