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89 lines
3.0 KiB
Markdown
89 lines
3.0 KiB
Markdown
---
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slug: /en/sql-reference/statements/select/union
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sidebar_label: UNION
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---
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# UNION Clause
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You can use `UNION` with explicitly specifying `UNION ALL` or `UNION DISTINCT`.
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If you don't specify `ALL` or `DISTINCT`, it will depend on the `union_default_mode` setting. The difference between `UNION ALL` and `UNION DISTINCT` is that `UNION DISTINCT` will do a distinct transform for union result, it is equivalent to `SELECT DISTINCT` from a subquery containing `UNION ALL`.
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You can use `UNION` 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` has a `Nullable` type field.
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Queries that are parts of `UNION` can be enclosed in round brackets. [ORDER BY](../../../sql-reference/statements/select/order-by.md) and [LIMIT](../../../sql-reference/statements/select/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` in a subquery in the [FROM](../../../sql-reference/statements/select/from.md) clause.
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If you use `UNION` without explicitly specifying `UNION ALL` or `UNION DISTINCT`, you can specify the union mode using the [union_default_mode](../../../operations/settings/settings.md#union-default-mode) setting. The setting values can be `ALL`, `DISTINCT` or an empty string. However, if you use `UNION` with `union_default_mode` setting to empty string, it will throw an exception. The following examples demonstrate the results of queries with different values setting.
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Query:
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```sql
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SET union_default_mode = 'DISTINCT';
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SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 2;
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```
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Result:
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```text
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┌─1─┐
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│ 1 │
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└───┘
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┌─1─┐
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│ 2 │
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└───┘
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┌─1─┐
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│ 3 │
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└───┘
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```
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Query:
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```sql
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SET union_default_mode = 'ALL';
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SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 2;
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```
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Result:
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```text
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┌─1─┐
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│ 1 │
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└───┘
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┌─1─┐
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│ 2 │
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└───┘
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┌─1─┐
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│ 2 │
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└───┘
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┌─1─┐
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│ 3 │
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└───┘
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```
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Queries that are parts of `UNION/UNION ALL/UNION DISTINCT` can be run simultaneously, and their results can be mixed together.
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**See Also**
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- [insert_null_as_default](../../../operations/settings/settings.md#insert_null_as_default) setting.
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- [union_default_mode](../../../operations/settings/settings.md#union-default-mode) setting.
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[Original article](https://clickhouse.com/docs/en/sql-reference/statements/select/union/) <!-- hide -->
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