# Dictionary key and fields The `` clause describes the dictionary key and fields available for queries. Overall structure: ```xml Id ... ``` Columns are described in the structure: - `` – [Key column](external_dicts_dict_structure.md#dicts-external_dicts_dict_structure-key). - `` – [Data column](external_dicts_dict_structure.md#dicts-external_dicts_dict_structure-attributes). There can be a large number of columns. ## Key ClickHouse supports the following types of keys: - Numeric key. UInt64. Defined in the tag `` . - Composite key. Set of values of different types. Defined in the tag `` . A structure can contain either `` or `` .
The key doesn't need to be defined separately in attributes.
### Numeric key Format: `UInt64`. Configuration example: ```xml 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#dicts-external_dicts_dict_layout) in this case must be `complex_key_hashed` or `complex_key_cache`.
A composite key can also consist of a single element, which makes it possible to use a string as the key, for instance.
The key structure is set in the element ``. Key fields are specified in the same format as the dictionary [attributes](external_dicts_dict_structure.md#dicts-external_dicts_dict_structure-attributes). Example: ```xml field1 String field2 UInt32 ... ... ``` 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 Configuration example: ```xml ... Name Type rand64() true true true ``` Configuration fields: - `name` – The column name. - `type` – The column type. Sets the method for interpreting data in the source. For example, for MySQL, the field might be `TEXT`, `VARCHAR`, or `BLOB` in the source table, but it can be uploaded as `String`. - `null_value` – The default value for a non-existing element. In the example, it is an empty string. - `expression` – The attribute can be an expression. The tag is not required. - `hierarchical` – Hierarchical support. Mirrored to the parent identifier. By default, ` false`. - `injective` Whether the `id -> attribute` image is injective. If ` true`, then you can optimize the ` GROUP BY` clause. By default, `false`. - `is_object_id` - Used for query mongo documents by ObjectId