SQL“分组依据”仅返回第一行-bis

Question

I understand from this: SQL group by returns just first row that MySQL returns only the first value found for a group.

To simplify I have the following tables:

Products
----------------
id | description
----------------
1  | Product-1
2  | Product-2
3  | Product-3


Groups
------
id | description
----------------
1  | Group-1
2  | Group-2


nxm_Products_Groups
---------------------
product_id | group_id | qty
---------------------
1          |   1      | 10
1          |   2      | 10
2          |   1      | 10
3          |   1      | 10

Last table is an associative table many-to-many between products and groups.

I want to group by the products, summing up their quantity ( qty ), and when all the products summed are in a single group, I want the group name selected, otherwise I want "??" returned.

MySQL always returns the first group it finds:

SELECT  
    G.id AS GROUP_ID,
    G.description,  
    P.id AS PRODUCT_ID,  
    P.description,  
    SUM(GP.qty) AS QTY 
 FROM  
    Groups G  
 JOIN nxm_Products_Groups GP ON (G.id = GP.group_id)  
 JOIN Products P ON (P.id = GP.product_id)  
 GROUP BY P.id  
 ORDER BY GP.group_id;

I get:

GROUP_ID|description|PRODUCT_ID|description|QTY
1       |Group 1    |2         |Product-2  |10
1       |Group 1    |3         |Product-3  |10
1       |Group 1    |1         |Product-1  |20

Edit: maybe my question was not clear. Instead of what I get, I want:

GROUP_ID|description|PRODUCT_ID|description|QTY
1       |Group 1    |2         |Product-2  |10
1       |Group 1    |3         |Product-3  |10
???     |???        |1         |Product-1  |20

How?

Answer 1

You can't use ??? instead of group_ID since it's an INT field. You can use for example 0 . Try to use CASE :

SELECT  
    CASE WHEN COUNT(*)=1 THEN G.id ELSE 0 END AS GROUP_ID,
    CASE WHEN COUNT(*)=1 THEN G.description ELSE '\?\?' END AS GROUP_DESC,  
    P.id AS PRODUCT_ID,  
    P.description,  
    SUM(GP.qty) AS QTY 
 FROM  
    Groups G  
 JOIN nxm_Products_Groups GP ON (G.id = GP.group_id)  
 JOIN Products P ON (P.id = GP.product_id)  
 GROUP BY P.id  
 ORDER BY GROUP_ID;

SQLFiddle demo