ClickHouse/docs/en/query_language/dicts/external_dicts_dict_hierarchical.md

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# Hierarchical Dictionaries
ClickHouse supports hierarchical dictionaries with a [numeric key](external_dicts_dict_structure.md#ext_dict-numeric-key).
Look at the following hierarchical structure:
```text
0 (Common parent)
├── 1 (Russia)
│ │
│ └── 2 (Moscow)
│ │
│ └── 3 (Center)
└── 4 (Great Britain)
└── 5 (London)
```
This hierarchy can be expressed as the following dictionary table.
region_id | parent_region | region_name
----|--------------|------
1 | 0 | Russia
2 | 1 | Moscow
3 | 2 | Center
4 | 0 | Great Britain
5 | 4 | London
This table contains a column `parent_region` that contains the key of the nearest parent for the element.
ClickHouse supports the [hierarchical](external_dicts_dict_structure.md#hierarchical-dict-attr) property for [external dictionary](index.md) attributes. This property allows you to configure the hierarchical dictionary similar to described above.
The [dictGetHierarchy](../functions/ext_dict_functions.md#dictgethierarchy) function allows you to get the parent chain of an element.
For our example, the structure of dictionary can be the following:
```xml
<dictionary>
<structure>
<id>
<name>region_id</name>
</id>
<attribute>
<name>parent_region</name>
<type>UInt64</type>
<null_value>0</null_value>
<hierarchical>true</hierarchical>
</attribute>
<attribute>
<name>region_name</name>
<type>String</type>
<null_value></null_value>
</attribute>
</structure>
</dictionary>
```
[Original article](https://clickhouse.tech/docs/en/query_language/dicts/external_dicts_dict_hierarchical/) <!--hide-->