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string_agg Function

The string_agg(value, delimiter) aggregate function concatenates the non-null input values (i.e. value) into text. Each value after the first is preceded by its corresponding delimiter, where null values are equivalent to an empty string. The input values to the aggregate can be filtered.

Syntax

string_agg ( value , delimiter ORDER BY col_ref ASC DESC , ) FILTER ( WHERE filter_clause )

Signatures

Parameter Type Description
value text The values to concatenate.
delimiter text The value to precede each concatenated value.

Return value

string_agg returns a text value.

This function always executes on the data from value as if it were sorted in ascending order before the function call. Any specified ordering is ignored. If you need to perform aggregation in a specific order, you must specify ORDER BY within the aggregate function call itself. Otherwise incoming rows are not guaranteed any order.

Usage in dataflows

While string_agg is available in Materialize, materializing views using it is considered an incremental view maintenance anti-pattern. Any change to the data underlying the function call will require the function to be recomputed entirely, discarding the benefits of maintaining incremental updates.

Instead, we recommend that you materialize all components required for the string_agg function call and create a non-materialized view using string_agg on top of that. That pattern is illustrated in the following statements:

CREATE MATERIALIZED VIEW foo_view AS SELECT * FROM foo;
CREATE VIEW bar AS SELECT string_agg(foo_view.bar, ',');

Examples

SELECT string_agg(column1, column2)
FROM (
    VALUES ('z', ' !'), ('a', ' @'), ('m', ' #')
);
 string_agg
------------
 a #m !z

Note that in the following example, the ORDER BY of the subquery feeding into string_agg gets ignored.

SELECT column1, column2
FROM (
    VALUES ('z', ' !'), ('a', ' @'), ('m', ' #')
) ORDER BY column1 DESC;
 column1 | column2
---------+---------
 z       |  !
 m       |  #
 a       |  @
SELECT string_agg(column1, column2)
FROM (
    SELECT column1, column2
    FROM (
        VALUES ('z', ' !'), ('a', ' @'), ('m', ' #')
    ) f ORDER BY column1 DESC
) g;
 string_agg
------------
 a #m !z
SELECT string_agg(b, ',' ORDER BY a DESC) FROM table;
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