MySQL-将值分组在一起的查询

Question

I have a query (below) which returns and ID value and two additional associated values (ReplacesReference). The query returns 2 records. I want to return one record so I need to concatenate the first "ReplacesReference" value with a comma ',' and the second "ReplacesReference" value.

my query is as follows

    select 
    distinct(c.wiid) as ID,    
    a.reference as ReplacesReference
    from npp_projects a, 
    npp_assoc b, 
    npp_projects c
    where a.id = b.parent_id     
    and b.type = 'replace' 
    and c.id = b.child_id
   and c.reference like '%EN 815%'

Its output is :

ID          |   ReplacesReference
====================================
CEN3406     |   I.S. EN 815:1996
CEN3406     |   I.S. EN 815:2004

I want to be able to return the below output:

ID                | ReplacesReference
====================================
CEN3406           |  I.S. EN 815:1996 , I.S. EN 815:2004

Many thanks in advance for your help - I am tearing my hair out with this one !

Answer 1

You'd have to do something like:

SELECT id, GROUP_CONCAT(string SEPARATOR ' ') FROM table GROUP BY id; http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat

Answer 2

You want to use group by instead of select distinct . Your use of parentheses around c.wild suggests that you don't understand how select distinct works. It applies to all columns in the select list. The parentheses don't affect it. The query looks like:

select p2.wiid as ID,    
       group_concat(distinct p.reference separator ', ') as ReplacesReference
from npp_projects p join
     npp_assoc a
     on p.id = a.parent_id   
     npp_projects p2
     on p2.id = a.child_id
where a.type = 'replace' and p2.reference like '%EN 815%'
group by p2.wild;

The changes I made:

• Added the group_concat() to get the list you want.
• Added the group by to replace the select distinct .
• Changed the table aliases to use table abbreviations so the query is easier to follow.
• Changed the join syntax to the more powerful explicit join syntax. As a simple rule: just do not use commas in the from clause.