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mosabua merged 1 commit into from
Dec 20, 2023
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Document filtering during aggregation for listagg #20162

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46 changes: 27 additions & 19 deletions docs/src/main/sphinx/functions/aggregate.md
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Original file line number Diff line number Diff line change
Expand Up @@ -180,7 +180,7 @@ Synopsis:

```
LISTAGG( expression [, separator] [ON OVERFLOW overflow_behaviour])
WITHIN GROUP (ORDER BY sort_item, [...])
WITHIN GROUP (ORDER BY sort_item, [...]) [FILTER (WHERE condition)]
```

If `separator` is not specified, the empty string will be used as `separator`.
Expand Down Expand Up @@ -241,28 +241,36 @@ results in:
200 | b,c
```

This aggregation function can be also used with the `FILTER` keyword to specify
which rows are processed during the `listagg` aggregation:
This aggregation function supports
[filtering during aggregation](aggregate-function-filtering-during-aggregation)
for scenarios where the aggregation for the data not matching the filter
condition still needs to show up in the output:

```sql
SELECT listagg(value, ',')
WITHIN GROUP (ORDER BY id)
FILTER (WHERE id % 2 = 0) csv_value
FROM (VALUES
(1, 'a'),
(2, 'b'),
(3, 'c'),
(4, 'd')
) t(id, value)
```
SELECT
country,
listagg(city, ',')
WITHIN GROUP (ORDER BY population DESC)
FILTER (WHERE population >= 10_000_000) megacities
FROM (VALUES
('India', 'Bangalore', 13_700_000),
('India', 'Chennai', 12_200_000),
('India', 'Ranchi', 1_547_000),
('Austria', 'Vienna', 1_897_000),
('Poland', 'Warsaw', 1_765_000)
) t(country, city, population)
GROUP BY country
ORDER BY country;
```

The example aggregates rows that have even-numbered `id`, and concatenates
`value` to a comma-separated string:
results in:

```
csv_value
-----------
b,d
```text
country | megacities
---------+-------------------
Austria | NULL
India | Bangalore,Chennai
Poland | NULL
```

The current implementation of `listagg` function does not support window frames.
Expand Down