I need to create a table with accumulated values

Question

I have this input table:

T1

f1 f2
1  S1
1  S2
1  S3
2  S4
3  S5
3  S6

And need to produce this output table:

T2

g1 g2
1  S1+S2+S3
2  S4
3  S5+S6

S# values are strings. g1 is the grouped by value of f1. g2 is the concatenation of values f2.

Can I write a Postgres SQL statement to achieve this?

Answer 1

Are you just looking for string_agg()?

select f1, string_agg(f2, '+')
from t1
group by f1;

I don't really recommend string_agg() unless you have a specific need for it. Arrays are more useful than strings:

select f1, array_agg(f2)
from t1
group by f1;

Also note that the ordering is not guaranteed for the elements in the string or array. You can add order by f2 if that is the ordering you want.

Answer 2

Use string_agg():

select f1, string_agg(f2, '+') f2s
from t1
group by f1