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157 lines
4.3 KiB
Markdown
157 lines
4.3 KiB
Markdown
---
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slug: /en/sql-reference/functions/array-join
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sidebar_position: 15
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sidebar_label: arrayJoin
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---
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# arrayJoin function
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This is a very unusual function.
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Normal functions do not change a set of rows, but just change the values in each row (map).
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Aggregate functions compress a set of rows (fold or reduce).
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The `arrayJoin` function takes each row and generates a set of rows (unfold).
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This function takes an array as an argument, and propagates the source row to multiple rows for the number of elements in the array.
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All the values in columns are simply copied, except the values in the column where this function is applied; it is replaced with the corresponding array value.
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Example:
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``` sql
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SELECT arrayJoin([1, 2, 3] AS src) AS dst, 'Hello', src
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```
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``` text
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┌─dst─┬─\'Hello\'─┬─src─────┐
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│ 1 │ Hello │ [1,2,3] │
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│ 2 │ Hello │ [1,2,3] │
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│ 3 │ Hello │ [1,2,3] │
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└─────┴───────────┴─────────┘
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```
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The `arrayJoin` function affects all sections of the query, including the `WHERE` section. Notice the result 2, even though the subquery returned 1 row.
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Example:
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```sql
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SELECT sum(1) AS impressions
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FROM
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(
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SELECT ['Istanbul', 'Berlin', 'Bobruisk'] AS cities
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)
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WHERE arrayJoin(cities) IN ['Istanbul', 'Berlin'];
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```
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``` text
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┌─impressions─┐
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│ 2 │
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└─────────────┘
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```
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A query can use multiple `arrayJoin` functions. In this case, the transformation is performed multiple times and the rows are multiplied.
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Example:
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```sql
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SELECT
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sum(1) AS impressions,
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arrayJoin(cities) AS city,
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arrayJoin(browsers) AS browser
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FROM
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(
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SELECT
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['Istanbul', 'Berlin', 'Bobruisk'] AS cities,
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['Firefox', 'Chrome', 'Chrome'] AS browsers
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)
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GROUP BY
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2,
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3
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```
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``` text
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┌─impressions─┬─city─────┬─browser─┐
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│ 2 │ Istanbul │ Chrome │
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│ 1 │ Istanbul │ Firefox │
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│ 2 │ Berlin │ Chrome │
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│ 1 │ Berlin │ Firefox │
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│ 2 │ Bobruisk │ Chrome │
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│ 1 │ Bobruisk │ Firefox │
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└─────────────┴──────────┴─────────┘
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```
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### Important note!
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Using multiple `arrayJoin` with same expression may not produce expected results due to optimizations.
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For that cases, consider modifying repeated array expression with extra operations that do not affect join result - e.g. `arrayJoin(arraySort(arr))`, `arrayJoin(arrayConcat(arr, []))`
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Example:
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```sql
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SELECT
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arrayJoin(dice) as first_throw,
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/* arrayJoin(dice) as second_throw */ -- is technically correct, but will annihilate result set
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arrayJoin(arrayConcat(dice, [])) as second_throw -- intentionally changed expression to force re-evaluation
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FROM (
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SELECT [1, 2, 3, 4, 5, 6] as dice
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);
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```
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Note the [ARRAY JOIN](../statements/select/array-join.md) syntax in the SELECT query, which provides broader possibilities.
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`ARRAY JOIN` allows you to convert multiple arrays with the same number of elements at a time.
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Example:
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```sql
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SELECT
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sum(1) AS impressions,
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city,
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browser
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FROM
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(
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SELECT
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['Istanbul', 'Berlin', 'Bobruisk'] AS cities,
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['Firefox', 'Chrome', 'Chrome'] AS browsers
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)
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ARRAY JOIN
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cities AS city,
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browsers AS browser
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GROUP BY
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2,
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3
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```
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``` text
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┌─impressions─┬─city─────┬─browser─┐
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│ 1 │ Istanbul │ Firefox │
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│ 1 │ Berlin │ Chrome │
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│ 1 │ Bobruisk │ Chrome │
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└─────────────┴──────────┴─────────┘
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```
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Or you can use [Tuple](../data-types/tuple.md)
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Example:
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```sql
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SELECT
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sum(1) AS impressions,
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(arrayJoin(arrayZip(cities, browsers)) AS t).1 AS city,
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t.2 AS browser
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FROM
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(
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SELECT
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['Istanbul', 'Berlin', 'Bobruisk'] AS cities,
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['Firefox', 'Chrome', 'Chrome'] AS browsers
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)
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GROUP BY
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2,
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3
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```
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``` text
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┌─impressions─┬─city─────┬─browser─┐
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│ 1 │ Istanbul │ Firefox │
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│ 1 │ Berlin │ Chrome │
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│ 1 │ Bobruisk │ Chrome │
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└─────────────┴──────────┴─────────┘
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```
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