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35 lines
1.4 KiB
Markdown
35 lines
1.4 KiB
Markdown
---
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toc_title: UNION ALL
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---
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# UNION ALL Clause {#union-all-clause}
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You can use `UNION ALL` to combine any number of `SELECT` queries by extending their results. Example:
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``` sql
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SELECT CounterID, 1 AS table, toInt64(count()) AS c
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FROM test.hits
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GROUP BY CounterID
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UNION ALL
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SELECT CounterID, 2 AS table, sum(Sign) AS c
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FROM test.visits
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GROUP BY CounterID
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HAVING c > 0
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```
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Result columns are matched by their index (order inside `SELECT`). If column names do not match, names for the final result are taken from the first query.
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Type casting is performed for unions. For example, if two queries being combined have the same field with non-`Nullable` and `Nullable` types from a compatible type, the resulting `UNION ALL` has a `Nullable` type field.
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Queries that are parts of `UNION ALL` can’t be enclosed in round brackets. [ORDER BY](order-by.md) and [LIMIT](limit.md) are applied to separate queries, not to the final result. If you need to apply a conversion to the final result, you can put all the queries with `UNION ALL` in a subquery in the [FROM](from.md) clause.
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## Limitations
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Only `UNION ALL` is supported. The regular `UNION` (`UNION DISTINCT`) is not supported. If you need `UNION DISTINCT`, you can write `SELECT DISTINCT` from a subquery containing `UNION ALL`.
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## Implementation Details
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Queries that are parts of `UNION ALL` can be run simultaneously, and their results can be mixed together.
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