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68 lines
2.1 KiB
Markdown
68 lines
2.1 KiB
Markdown
---
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toc_priority: 45
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toc_title: Hierarchical dictionaries
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---
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# Hierarchical Dictionaries {#hierarchical-dictionaries}
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ClickHouse supports hierarchical dictionaries with a [numeric key](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict-numeric-key).
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Look at the following hierarchical structure:
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``` text
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0 (Common parent)
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│
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├── 1 (Russia)
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│ │
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│ └── 2 (Moscow)
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│ │
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│ └── 3 (Center)
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│
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└── 4 (Great Britain)
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│
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└── 5 (London)
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```
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This hierarchy can be expressed as the following dictionary table.
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| region_id | parent_region | region_name |
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|------------|----------------|---------------|
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| 1 | 0 | Russia |
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| 2 | 1 | Moscow |
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| 3 | 2 | Center |
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| 4 | 0 | Great Britain |
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| 5 | 4 | London |
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This table contains a column `parent_region` that contains the key of the nearest parent for the element.
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ClickHouse supports the [hierarchical](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#hierarchical-dict-attr) property for [external dictionary](../../../sql-reference/dictionaries/external-dictionaries/index.md) attributes. This property allows you to configure the hierarchical dictionary similar to described above.
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The [dictGetHierarchy](../../../sql-reference/functions/ext-dict-functions.md#dictgethierarchy) function allows you to get the parent chain of an element.
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For our example, the structure of dictionary can be the following:
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``` xml
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<dictionary>
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<structure>
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<id>
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<name>region_id</name>
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</id>
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<attribute>
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<name>parent_region</name>
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<type>UInt64</type>
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<null_value>0</null_value>
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<hierarchical>true</hierarchical>
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</attribute>
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<attribute>
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<name>region_name</name>
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<type>String</type>
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<null_value></null_value>
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</attribute>
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</structure>
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</dictionary>
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```
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