Google Spreadsheets: How do you concat strings in an aggregation function - google-sheets

Question

Say I have a table:
A, 1
B, 1
C, 2
D, 1
E, 2
How do I view the table grouping by the 2nd column and aggregating by the first with a comma separated concat function ie:
1, "A,B,D"
2, "C,E"
In both defining a pivot table and using the QUERY syntax, it seems that the only aggregation functions available are numerical aggregations like MIN, MAX, SUM, etc. Can I define my own aggregation function?

Answer 1

You have to add a "Calculated Field" to the pivot table, and then select "Summarise by > Custom". This will make the column names in your formula refer to an array of values (instead of a single value). Then you can type a formula like:
= JOIN(", ", MyStringColumn)
More specifically, if you have the following table:
Create a pivot table by going to "Data > Pivot table", with the following configuration. Ensure "Summarize by" is set to "Custom"!

Answer 2

Another option: if the data is in A2:B, then, say, in D2:
=UNIQUE(B2:B)
and then in E2:
=JOIN(",",FILTER(A$2:A,B$2:B=D2))
which is filled down as required.
There are one-formula, auto-expanding solutions, although they get quite convoluted.

Answer 3

You're right, there's no easy way with pivot tables. This though, will do the trick. Inspired by this brilliant answer here.
First, have a header row and run a sort on column A to group by category.
So far, in your example, we have
| A | B
---+-----------+-----------
1 | CATEGORY | ATTRIBUTE
2 | 1 | A
3 | 1 | B
4 | 1 | D
5 | 2 | C
6 | 2 | E
In column C, let's prep the concatenated strings. Start in cell C2 with the following formula, and fill out vertically.
=IF(A2<>A1, B2, C1 & "," & B2)
...looking good...
| A | B | C
---+-----------+-----------+-----------
1 | CATEGORY | ATTRIBUTE | STRINGS
2 | 1 | A | A
3 | 1 | B | A,B
4 | 1 | D | A,B,D
5 | 2 | C | C
6 | 2 | E | C,E
In column D, let's validate the rows we want to select in a later step, with the following formula, starting in cell D2 and filling out. Basically we are marking the final category rows that carry the full concatenated strings.
=A2<>A3
...almost there now
| A | B | C | D
---+-----------+-----------+----------+-----------
1 | CATEGORY | ATTRIBUTE | STRINGS | VALIDATOR
2 | 1 | A | A | FALSE
3 | 1 | B | A,B | FALSE
4 | 1 | D | A,B,D | TRUE
5 | 2 | C | C | FALSE
6 | 2 | E | C,E | TRUE
Now, lets copy column C and D and paste special as values in the same place. Then add a filter on the whole table and filter out column D for the rows labeled TRUE. Now, remove the filter, delete columns B and D and row 1.
| A | B
---+-----------+-----------
1 | 1 | A,B,D
2 | 2 | C,E
Done. Get ice cream. Watch Road House.