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45 lines
1.3 KiB
Markdown
45 lines
1.3 KiB
Markdown
---
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sidebar_position: 50
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sidebar_label: VIEW
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---
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# ALTER TABLE … MODIFY QUERY 语句 {#alter-modify-query}
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当使用`ALTER TABLE … MODIFY QUERY`语句创建一个[物化视图](../create/view.md#materialized)时，可以修改`SELECT`查询。当物化视图在没有 `TO [db.]name` 的情况下创建时使用它。必须启用 `allow_experimental_alter_materialized_view_structure`设置。
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如果一个物化视图使用`TO [db.]name`，你必须先 [DETACH](../detach.md) 视图。用[ALTER TABLE](index.md)修改目标表，然后 [ATTACH](../attach.md)之前分离的(`DETACH`)视图。
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**示例**
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```sql
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CREATE TABLE src_table (`a` UInt32) ENGINE = MergeTree ORDER BY a;
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CREATE MATERIALIZED VIEW mv (`a` UInt32) ENGINE = MergeTree ORDER BY a AS SELECT a FROM src_table;
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INSERT INTO src_table (a) VALUES (1), (2);
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SELECT * FROM mv;
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```
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```text
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┌─a─┐
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│ 1 │
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│ 2 │
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└───┘
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```
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```sql
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ALTER TABLE mv MODIFY QUERY SELECT a * 2 as a FROM src_table;
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INSERT INTO src_table (a) VALUES (3), (4);
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SELECT * FROM mv;
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```
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```text
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┌─a─┐
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│ 6 │
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│ 8 │
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└───┘
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┌─a─┐
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│ 1 │
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│ 2 │
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└───┘
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```
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## ALTER LIVE VIEW 语句 {#alter-live-view}
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`ALTER LIVE VIEW ... REFRESH` 语句刷新一个 [实时视图](../create/view.md#live-view). 参见 [强制实时视图刷新](../create/view.md#live-view-alter-refresh).
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