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31 lines
1.1 KiB
Markdown
31 lines
1.1 KiB
Markdown
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<a name="functions_arrayjoin"></a>
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# arrayJoin function
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This is a very unusual function.
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Normal functions don't 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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A query can use multiple `arrayJoin` functions. In this case, the transformation is performed multiple times.
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Note the ARRAY JOIN syntax in the SELECT query, which provides broader possibilities.
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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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