ClickHouse/docs/en/sql-reference/functions/ext-dict-functions.md

toc_priority	toc_title
58	Working with External Dictionaries

Functions for Working with External Dictionaries

For information on connecting and configuring external dictionaries, see External dictionaries.

dictGet

Retrieves a value from an external dictionary.

dictGet('dict_name', 'attr_name', id_expr)
dictGetOrDefault('dict_name', 'attr_name', id_expr, default_value_expr)

Parameters

• dict_name — Name of the dictionary. String literal.
• attr_name — Name of the column of the dictionary. String literal.
• id_expr — Key value. Expression returning a UInt64 or Tuple-type value depending on the dictionary configuration.
• default_value_expr — Value returned if the dictionary doesn’t contain a row with the id_expr key. Expression returning the value in the data type configured for the attr_name attribute.

Returned value

• If ClickHouse parses the attribute successfully in the attribute’s data type, functions return the value of the dictionary attribute that corresponds to id_expr.

• If there is no the key, corresponding to id_expr, in the dictionary, then:

  - `dictGet` returns the content of the `<null_value>` element specified for the attribute in the dictionary configuration.
  - `dictGetOrDefault` returns the value passed as the `default_value_expr` parameter.
  

ClickHouse throws an exception if it cannot parse the value of the attribute or the value doesn’t match the attribute data type.

Example

Create a text file ext-dict-text.csv containing the following:

1,1
2,2

The first column is id, the second column is c1.

Configure the external dictionary:

<yandex>
    <dictionary>
        <name>ext-dict-test</name>
        <source>
            <file>
                <path>/path-to/ext-dict-test.csv</path>
                <format>CSV</format>
            </file>
        </source>
        <layout>
            <flat />
        </layout>
        <structure>
            <id>
                <name>id</name>
            </id>
            <attribute>
                <name>c1</name>
                <type>UInt32</type>
                <null_value></null_value>
            </attribute>
        </structure>
        <lifetime>0</lifetime>
    </dictionary>
</yandex>

Perform the query:

SELECT
    dictGetOrDefault('ext-dict-test', 'c1', number + 1, toUInt32(number * 10)) AS val,
    toTypeName(val) AS type
FROM system.numbers
LIMIT 3

┌─val─┬─type───┐
│   1 │ UInt32 │
│   2 │ UInt32 │
│  20 │ UInt32 │
└─────┴────────┘

See Also

• External Dictionaries

dictHas

Checks whether a key is present in a dictionary.

dictHas('dict_name', id_expr)

Parameters

• dict_name — Name of the dictionary. String literal.
• id_expr — Key value. Expression returning a UInt64-type value.

Returned value

• 0, if there is no key.
• 1, if there is a key.

Type: UInt8.

dictGetHierarchy

Creates an array, containing all the parents of a key in the hierarchical dictionary.

Syntax

dictGetHierarchy('dict_name', key)

Parameters

• dict_name — Name of the dictionary. String literal.
• key — Key value. Expression returning a UInt64-type value.

Returned value

• Parents for the key.

Type: Array(UInt64).

dictIsIn

Checks the ancestor of a key through the whole hierarchical chain in the dictionary.

dictIsIn('dict_name', child_id_expr, ancestor_id_expr)

Parameters

• dict_name — Name of the dictionary. String literal.
• child_id_expr — Key to be checked. Expression returning a UInt64-type value.
• ancestor_id_expr — Alleged ancestor of the child_id_expr key. Expression returning a UInt64-type value.

Returned value

• 0, if child_id_expr is not a child of ancestor_id_expr.
• 1, if child_id_expr is a child of ancestor_id_expr or if child_id_expr is an ancestor_id_expr.

Type: UInt8.

Other Functions

ClickHouse supports specialized functions that convert dictionary attribute values to a specific data type regardless of the dictionary configuration.

Functions:

• dictGetInt8, dictGetInt16, dictGetInt32, dictGetInt64
• dictGetUInt8, dictGetUInt16, dictGetUInt32, dictGetUInt64
• dictGetFloat32, dictGetFloat64
• dictGetDate
• dictGetDateTime
• dictGetUUID
• dictGetString

All these functions have the OrDefault modification. For example, dictGetDateOrDefault.

Syntax:

dictGet[Type]('dict_name', 'attr_name', id_expr)
dictGet[Type]OrDefault('dict_name', 'attr_name', id_expr, default_value_expr)

Parameters

• dict_name — Name of the dictionary. String literal.
• attr_name — Name of the column of the dictionary. String literal.
• id_expr — Key value. Expression returning a UInt64-type value.
• default_value_expr — Value which is returned if the dictionary doesn’t contain a row with the id_expr key. Expression returning a value in the data type configured for the attr_name attribute.

Returned value

• If ClickHouse parses the attribute successfully in the attribute’s data type, functions return the value of the dictionary attribute that corresponds to id_expr.

• If there is no requested id_expr in the dictionary then:

  - `dictGet[Type]` returns the content of the `<null_value>` element specified for the attribute in the dictionary configuration.
  - `dictGet[Type]OrDefault` returns the value passed as the `default_value_expr` parameter.
  

ClickHouse throws an exception if it cannot parse the value of the attribute or the value doesn’t match the attribute data type.

Original article