--- toc_priority: 44 toc_title: Dictionary Key and Fields --- # Dictionary Key and Fields {#dictionary-key-and-fields} The `` clause describes the dictionary key and fields available for queries. XML description: ``` xml Id ... ``` Attributes are described in the elements: - `` — [Key column](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-key). - `` — [Data column](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-attributes). There can be a multiple number of attributes. DDL query: ``` sql CREATE DICTIONARY dict_name ( Id UInt64, -- attributes ) PRIMARY KEY Id ... ``` Attributes are described in the query body: - `PRIMARY KEY` — [Key column](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-key) - `AttrName AttrType` — [Data column](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-attributes). There can be a multiple number of attributes. ## Key {#ext_dict_structure-key} ClickHouse supports the following types of keys: - Numeric key. `UInt64`. Defined in the `` tag or using `PRIMARY KEY` keyword. - Composite key. Set of values of different types. Defined in the tag `` or `PRIMARY KEY` keyword. An xml structure can contain either `` or ``. DDL-query must contain single `PRIMARY KEY`. !!! warning "Warning" You must not describe key as an attribute. ### Numeric Key {#ext_dict-numeric-key} Type: `UInt64`. Configuration example: ``` xml Id ``` Configuration fields: - `name` – The name of the column with keys. For DDL-query: ``` sql CREATE DICTIONARY ( Id UInt64, ... ) PRIMARY KEY Id ... ``` - `PRIMARY KEY` – The name of the column with keys. ### Composite Key {#composite-key} The key can be a `tuple` from any types of fields. The [layout](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-layout.md) in this case must be `complex_key_hashed` or `complex_key_cache`. !!! tip "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 fields are specified in the same format as the dictionary [attributes](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md). Example: ``` xml field1 String field2 UInt32 ... ... ``` or ``` sql CREATE DICTIONARY ( field1 String, field2 String ... ) PRIMARY KEY field1, field2 ... ``` 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 ... Name ClickHouseDataType rand64() true true true ``` or ``` sql CREATE DICTIONARY somename ( Name ClickHouseDataType DEFAULT '' EXPRESSION rand64() HIERARCHICAL INJECTIVE IS_OBJECT_ID ) ``` Configuration fields: | Tag | Description | Required | |------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------| | `name` | Column name. | Yes | | `type` | ClickHouse data type: [UInt8](../../../sql-reference/data-types/int-uint.md), [UInt16](../../../sql-reference/data-types/int-uint.md), [UInt32](../../../sql-reference/data-types/int-uint.md), [UInt64](../../../sql-reference/data-types/int-uint.md), [Int8](../../../sql-reference/data-types/int-uint.md), [Int16](../../../sql-reference/data-types/int-uint.md), [Int32](../../../sql-reference/data-types/int-uint.md), [Int64](../../../sql-reference/data-types/int-uint.md), [Float32](../../../sql-reference/data-types/float.md), [Float64](../../../sql-reference/data-types/float.md), [UUID](../../../sql-reference/data-types/uuid.md), [Decimal32](../../../sql-reference/data-types/decimal.md), [Decimal64](../../../sql-reference/data-types/decimal.md), [Decimal128](../../../sql-reference/data-types/decimal.md), [Decimal256](../../../sql-reference/data-types/decimal.md), [String](../../../sql-reference/data-types/string.md), [Array](../../../sql-reference/data-types/array.md).
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.
[Nullable](../../../sql-reference/data-types/nullable.md) is currently supported for [Flat](external-dicts-dict-layout.md#flat), [Hashed](external-dicts-dict-layout.md#dicts-external_dicts_dict_layout-hashed), [ComplexKeyHashed](external-dicts-dict-layout.md#complex-key-hashed), [Direct](external-dicts-dict-layout.md#direct), [ComplexKeyDirect](external-dicts-dict-layout.md#complex-key-direct), [RangeHashed](external-dicts-dict-layout.md#range-hashed), [Polygon](external-dicts-dict-polygon.md), [Cache](external-dicts-dict-layout.md#cache), [ComplexKeyCache](external-dicts-dict-layout.md#complex-key-cache), [SSDCache](external-dicts-dict-layout.md#ssd-cache), [SSDComplexKeyCache](external-dicts-dict-layout.md#complex-key-ssd-cache) dictionaries. In [IPTrie](external-dicts-dict-layout.md#ip-trie) dictionaries `Nullable` types are not supported. | Yes | | `null_value` | Default value for a non-existing element.
In the example, it is an empty string. [NULL](../../syntax.md#null-literal) value can be used only for the `Nullable` types (see the previous line with types description). | Yes | | `expression` | [Expression](../../../sql-reference/syntax.md#syntax-expressions) that ClickHouse executes on the value.
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.

Default value: no expression. | No | | `hierarchical` | If `true`, the attribute contains the value of a parent key for the current key. See [Hierarchical Dictionaries](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-hierarchical.md).

Default value: `false`. | No | | `injective` | Flag that shows whether the `id -> attribute` image is [injective](https://en.wikipedia.org/wiki/Injective_function).
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.

Default value: `false`. | No | | `is_object_id` | Flag that shows whether the query is executed for a MongoDB document by `ObjectID`.

Default value: `false`. | No | **See Also** - [Functions for working with external dictionaries](../../../sql-reference/functions/ext-dict-functions.md).