Question

Say I have 2 tables, A and B, each A entity can possibly have multiple B entities, in one case if I want to get all B’s of some certain A’s, I might do it with a simple left join

select A.id aid,B.id bid from A left join B on B.aid = A.id where A.id = 1 

and it will return a result set like

aid   bid 1     1 1     2 1     3 

As you can see for the first column, all those 1’s are kinda duplicates. Is it possible to modify the SQL statement to let him return a result like

aid    bid  1      1,2,3 

in other words to link all the bid’s together as one entity?

Also what if there’s another table C, and each A can have multiple C’s, how to I make the SQL return a result set like

 aid    bid    cid   1     1,2,3   1,2 

instead of

 aid     bid    cid   1       1      1   1       2      1   1       3      1   1       1      2   1       2      2   1       3      2  

Thank you very much!

Answer 1

What DBMS are you using?

I can’t speak for others, but in MySQL, starting from 4.1, you can use GROUP_CONCAT

http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat

EG:

select A.id aid, GROUP_CONCAT(DISTINCT B.id) bid from A left join B on B.aid = A.id where A.id = 1 GROUP BY a.id