Join multiple rows into a string

Question

I have 2 tables in Oracle: table1 and table2 (we have a very unimaginative DBA!)

table1 has column id (and a few others).

table2 has columns id, table1id and code. table1id is a foreign key to table1.

Usually there is 1 table2 row for each table1 row, but sometimes there are 2 table2 rows for a table1 row, and very occasionally 3.

What I need is a single 'code' value for each table1 row. If there is more than one coresponding table2 row, I need a concatenated string of all values returned, so the result might look like this:

table1.id     code
1             a
2             b
3             a b
4             a b c

Any idea how this can be achieved? If it were SQL Server I'd write a function, although this would be slower than I would like. However I'm not as hot in Oracle as I am in SQL Server.

Answer 1

You did not specify what version of Oracle but. If you are using Oracle 11g, then you can use the LISTAGG() function:

select t1.id,
    listagg(t2.code, ' ') within group(order by t1.id) code
from table1 t1
left join table2 t2
    on t1.id = t2.id
group by t1.id

If you are using Oracle 10g then you can use WM_CONCAT:

select t1.id,
    WM_CONCAT(t2.code) code
from table1 t1
left join table2 t2
    on t1.id = t2.id
group by t1.id

Answer 2

@bluefeet has the correct solution, but if you are in 10g and don't want to use WM_CONCAT for some reason(for exemple, it's not documented), there is another one:

select t1.id, 
   RTRIM(XMLAGG(XMLELEMENT(e,code,',').EXTRACT('//text()')),',') code 
from table1 t1
left join table2 t2 
   on t1.id = t2.id
group by t1.id