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sqlpostgresqlaggregate-functions

Concatenate multiple result rows of one column into one, group by another column

I'm having a table like this

Movie Actor
A 1
A 2
A 3
B 4

I want to get the name of a movie and all actors in that movie, and I want the result to be in a format like this:

Movie ActorList
A 1, 2, 3

How can I do it?

Solution

  • Simpler with the aggregate function string_agg() (Postgres 9.0 or later):

    SELECT movie, string_agg(actor, ', ') AS actor_list
FROM   tbl
GROUP  BY 1;

    The 1 in GROUP BY 1 is a positional reference and a shortcut for GROUP BY movie in this case.

    string_agg() expects data type text as input. Other types need to be cast explicitly (actor::text) - unless an implicit cast to text is defined - which is the case for all other string types (varchar, character, name, ...) and some other types.

    As isapir commented, you can add an ORDER BY clause in the aggregate call to get a sorted list - should you need that. Like:

    SELECT movie, string_agg(actor, ', ' ORDER BY actor) AS actor_list
FROM   tbl
GROUP  BY 1;

    But it's typically faster to sort rows in a subquery. See: