Mattijn Mattijn - 1 year ago 76
SQL Question

How to aggregate two PostgreSQL columns to an array separated by brackets

I would like to concatenate two columns using a group-by query resulting in an array separed with brackets. I know this question is related to this question, but as usual my use-case is a little different.

A simple example (also as SQL Fiddle).
Currently my query returns the following:

3 0.5 2.71
3 1.0 2.50
3 1.5 2.33
6 0.5 2.73
6 1.5 2.77

But where I would like concatenate/aggregate the
columns to get the following:

3 [[0.5,2.71],[1.0,2.50],[1.5,2.33]]
6 [[0.5,2.73],[1.5,2.77]]

Currently I've tried to concatenate the columns into one as follows:

SELECT "ID",concat_ws(', ',"X", "Y") as XY FROM Table1;

Which returns:

ID xy
3 0.5, 2.71
3 1, 2.50
3 1.5, 2.33
6 0.5, 2.73

And used

SELECT "ID",array_to_string(array_agg("X"),',') AS XY
FROM Table1

Resulting in:

ID xy
3 0.5,1,1.5
6 0.5

I feel I'm getting closer, but a helping hand would be really appreciated.

Answer Source

Create an array from the two columns, the aggregate the array:

select id, array_agg(array[x,y])
from the_table
group by id;

Note that the default text representation of arrays uses curly braces ( {..}) not square brackets ([..])