# Dictionary Key and Fields

The `<structure>` clause describes the dictionary key and fields available for queries.

Overall structure:

```xml
<dictionary>
    <structure>
        <id>
            <name>Id</name>
        </id>

        <attribute>
            <!-- Attribute parameters -->
        </attribute>

        ...

    </structure>
</dictionary>
```

Columns are described in the structure:

- `<id>` — [Key column](external_dicts_dict_structure.md#ext_dict_structure-key).
- `<attribute>` — [Data column](external_dicts_dict_structure.md#ext_dict_structure-attributes). There can be a large number of columns.


## Key {#ext_dict_structure-key}

ClickHouse supports the following types of keys:

- Numeric key. UInt64. Defined in the tag `<id>` .
- Composite key. Set of values of different types. Defined in the tag `<key>` .

A structure can contain either `<id>` or `<key>` .

!!! warning
    The key doesn't need to be defined separately in attributes.

### Numeric Key

Type: `UInt64`.

Configuration example:

```xml
<id>
    <name>Id</name>
</id>
```

Configuration fields:

- `name` – The name of the column with keys.

### Composite Key

The key can be a `tuple` from any types of fields. The [layout](external_dicts_dict_layout.md) in this case must be `complex_key_hashed` or `complex_key_cache`.

!!! tip
    A composite key can consist of a single element. This makes it possible to use a string as the key, for instance.

The key structure is set in the element `<key>`. Key fields are specified in the same format as the dictionary [attributes](external_dicts_dict_structure.md). Example:

```xml
<structure>
    <key>
        <attribute>
            <name>field1</name>
            <type>String</type>
        </attribute>
        <attribute>
            <name>field2</name>
            <type>UInt32</type>
        </attribute>
        ...
    </key>
...
```

For a query to the `dictGet*` function, a tuple is passed as the key. Example: `dictGetString('dict_name', 'attr_name', tuple('string for field1', num_for_field2))`.


## Attributes {#ext_dict_structure-attributes}

Configuration example:

```xml
<structure>
    ...
    <attribute>
        <name>Name</name>
        <type>ClickHouseDataType</type>
        <null_value></null_value>
        <expression>rand64()</expression>
        <hierarchical>true</hierarchical>
        <injective>true</injective>
        <is_object_id>true</is_object_id>
    </attribute>
</structure>
```

Configuration fields:

Tag | Description | Required
----|-------------|---------
`name`| Column name. | Yes
`type`| ClickHouse data type.<br/>ClickHouse tries to cast value from dictionary to the specified data type. For example, for MySQL, the field might be `TEXT`, `VARCHAR`, or `BLOB` in the MySQL source table, but it can be uploaded as `String` in ClickHouse.<br/>[Nullable](../../data_types/nullable.md) is not supported. | Yes
`null_value` | Default value for a non-existing element.<br/>In the example, it is an empty string. You cannot use `NULL` in this field. | Yes
`expression` | [Expression](../syntax.md#syntax-expressions) that ClickHouse executes on the value.<br/>The expression can be a column name in the remote SQL database. Thus, you can use it to create an alias for the remote column.<br/><br/>Default value: no expression. | No
`hierarchical` | Hierarchical support. Mirrored to the parent identifier.<br/><br/>Default value: `false`. | No
`injective` | Flag that shows whether the `id -> attribute` image is [injective](https://en.wikipedia.org/wiki/Injective_function).<br/>If `true`, ClickHouse can automatically place after the `GROUP BY` clause the requests to dictionaries with injection. Usually it significantly reduces the amount of such requests.<br/><br/>Default value: `false`. | No
`is_object_id` | Flag that shows whether the query is executed for a MongoDB document by `ObjectID`.<br/><br/>Default value: `false`. | No

[Original article](https://clickhouse.yandex/docs/en/query_language/dicts/external_dicts_dict_structure/) <!--hide-->