# 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-->