GPT4All-J-v1.3 Groovy [1] gave me the following answer (no idea if this is good or not, but keep in mind that the model comes in a 3.8Gb file and is released under an Apache 2 license, freely available for use and distribution):

    To join a column with SQL in Postgres to a string separated by a comma, you can use the STRING_AGG function. Here's an example query that demonstrates this:
    SELECT STRING_AGG(column_name, ', ') WITHIN GROUP (ORDER BY id) AS joined_string FROM table;
    In this query, we're selecting the joined_string column from a table called table. The STRING_AGG function is used to concatenate all values in the column_name column into a single string separated by commas. We use the WITHIN GROUP (ORDER BY id) clause to specify that we want to group the results by an integer value called id. This ensures that each row of data is treated as a separate entity, even if it has the same column_name.
    The resulting output will be a single column with all values from the joined_string column concatenated into a string separated by commas. You can then use this query in your application to join multiple columns together and store them as a single string value.

[1] https://gpt4all.io/reports/GPT4All_Technical_Report_3.pdf