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ClickHouse/docs/en/sql-reference/functions/ext-dict-functions.md
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---
sidebar_position: 58
sidebar_label: External Dictionaries
---
:::note
For dictionaries created with [DDL queries](../../sql-reference/statements/create/dictionary.md), the `dict_name` parameter must be fully specified, like `<database>.<dict_name>`. Otherwise, the current database is used.
:::
# Functions for Working with External Dictionaries
For information on connecting and configuring external dictionaries, see [External dictionaries](../../sql-reference/dictionaries/external-dictionaries/external-dicts.md).
## dictGet, dictGetOrDefault, dictGetOrNull
Retrieves values from an external dictionary.
``` sql
dictGet('dict_name', attr_names, id_expr)
dictGetOrDefault('dict_name', attr_names, id_expr, default_value_expr)
dictGetOrNull('dict_name', attr_name, id_expr)
```
**Arguments**
- `dict_name` — Name of the dictionary. [String literal](../../sql-reference/syntax.md#syntax-string-literal).
- `attr_names` — Name of the column of the dictionary, [String literal](../../sql-reference/syntax.md#syntax-string-literal), or tuple of column names, [Tuple](../../sql-reference/data-types/tuple.md)([String literal](../../sql-reference/syntax.md#syntax-string-literal)).
- `id_expr` — Key value. [Expression](../../sql-reference/syntax.md#syntax-expressions) returning dictionary key-type value or [Tuple](../../sql-reference/data-types/tuple.md)-type value depending on the dictionary configuration.
- `default_value_expr` — Values returned if the dictionary does not contain a row with the `id_expr` key. [Expression](../../sql-reference/syntax.md#syntax-expressions) or [Tuple](../../sql-reference/data-types/tuple.md)([Expression](../../sql-reference/syntax.md#syntax-expressions)), returning the value (or values) in the data types configured for the `attr_names` attribute.
**Returned value**
- If ClickHouse parses the attribute successfully in the [attribute’s data type](../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-attributes), 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.
- `dictGetOrNull` returns `NULL` in case key was not found in dictionary.
ClickHouse throws an exception if it cannot parse the value of the attribute or the value does not match the attribute data type.
**Example for simple key dictionary**
Create a text file `ext-dict-test.csv` containing the following:
``` text
1,1
2,2
```
The first column is `id`, the second column is `c1`.
Configure the external dictionary:
``` xml
<clickhouse>
<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>
</clickhouse>
```
Perform the query:
``` sql
SELECT
dictGetOrDefault('ext-dict-test', 'c1', number + 1, toUInt32(number * 10)) AS val,
toTypeName(val) AS type
FROM system.numbers
LIMIT 3;
```
``` text
┌─val─┬─type───┐
│ 1 │ UInt32 │
│ 2 │ UInt32 │
│ 20 │ UInt32 │
└─────┴────────┘
```
**Example for complex key dictionary**
Create a text file `ext-dict-mult.csv` containing the following:
``` text
1,1,'1'
2,2,'2'
3,3,'3'
```
The first column is `id`, the second is `c1`, the third is `c2`.
Configure the external dictionary:
``` xml
<clickhouse>
<dictionary>
<name>ext-dict-mult</name>
<source>
<file>
<path>/path-to/ext-dict-mult.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>
<attribute>
<name>c2</name>
<type>String</type>
<null_value></null_value>
</attribute>
</structure>
<lifetime>0</lifetime>
</dictionary>
</clickhouse>
```
Perform the query:
``` sql
SELECT
dictGet('ext-dict-mult', ('c1','c2'), number) AS val,
toTypeName(val) AS type
FROM system.numbers
LIMIT 3;
```
``` text
┌─val─────┬─type──────────────────┐
│ (1,'1') │ Tuple(UInt8, String) │
│ (2,'2') │ Tuple(UInt8, String) │
│ (3,'3') │ Tuple(UInt8, String) │
└─────────┴───────────────────────┘
```
**Example for range key dictionary**
Input table:
```sql
CREATE TABLE range_key_dictionary_source_table
(
key UInt64,
start_date Date,
end_date Date,
value String,
value_nullable Nullable(String)
)
ENGINE = TinyLog();
INSERT INTO range_key_dictionary_source_table VALUES(1, toDate('2019-05-20'), toDate('2019-05-20'), 'First', 'First');
INSERT INTO range_key_dictionary_source_table VALUES(2, toDate('2019-05-20'), toDate('2019-05-20'), 'Second', NULL);
INSERT INTO range_key_dictionary_source_table VALUES(3, toDate('2019-05-20'), toDate('2019-05-20'), 'Third', 'Third');
```
Create the external dictionary:
```sql
CREATE DICTIONARY range_key_dictionary
(
key UInt64,
start_date Date,
end_date Date,
value String,
value_nullable Nullable(String)
)
PRIMARY KEY key
SOURCE(CLICKHOUSE(HOST 'localhost' PORT tcpPort() TABLE 'range_key_dictionary_source_table'))
LIFETIME(MIN 1 MAX 1000)
LAYOUT(RANGE_HASHED())
RANGE(MIN start_date MAX end_date);
```
Perform the query:
``` sql
SELECT
(number, toDate('2019-05-20')),
dictHas('range_key_dictionary', number, toDate('2019-05-20')),
dictGetOrNull('range_key_dictionary', 'value', number, toDate('2019-05-20')),
dictGetOrNull('range_key_dictionary', 'value_nullable', number, toDate('2019-05-20')),
dictGetOrNull('range_key_dictionary', ('value', 'value_nullable'), number, toDate('2019-05-20'))
FROM system.numbers LIMIT 5 FORMAT TabSeparated;
```
Result:
``` text
(0,'2019-05-20') 0 \N \N (NULL,NULL)
(1,'2019-05-20') 1 First First ('First','First')
(2,'2019-05-20') 1 Second \N ('Second',NULL)
(3,'2019-05-20') 1 Third Third ('Third','Third')
(4,'2019-05-20') 0 \N \N (NULL,NULL)
```
**See Also**
- [External Dictionaries](../../sql-reference/dictionaries/external-dictionaries/external-dicts.md)
## dictHas
Checks whether a key is present in a dictionary.
``` sql
dictHas('dict_name', id_expr)
```
**Arguments**
- `dict_name` — Name of the dictionary. [String literal](../../sql-reference/syntax.md#syntax-string-literal).
- `id_expr` — Key value. [Expression](../../sql-reference/syntax.md#syntax-expressions) returning dictionary key-type value or [Tuple](../../sql-reference/data-types/tuple.md)-type value depending on the dictionary configuration.
**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](../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-hierarchical.md).
**Syntax**
``` sql
dictGetHierarchy('dict_name', key)
```
**Arguments**
- `dict_name` — Name of the dictionary. [String literal](../../sql-reference/syntax.md#syntax-string-literal).
- `key` — Key value. [Expression](../../sql-reference/syntax.md#syntax-expressions) returning a [UInt64](../../sql-reference/data-types/int-uint.md)-type value.
**Returned value**
- Parents for the key.
Type: [Array(UInt64)](../../sql-reference/data-types/array.md).
## dictIsIn
Checks the ancestor of a key through the whole hierarchical chain in the dictionary.
``` sql
dictIsIn('dict_name', child_id_expr, ancestor_id_expr)
```
**Arguments**
- `dict_name` — Name of the dictionary. [String literal](../../sql-reference/syntax.md#syntax-string-literal).
- `child_id_expr` — Key to be checked. [Expression](../../sql-reference/syntax.md#syntax-expressions) returning a [UInt64](../../sql-reference/data-types/int-uint.md)-type value.
- `ancestor_id_expr` — Alleged ancestor of the `child_id_expr` key. [Expression](../../sql-reference/syntax.md#syntax-expressions) returning a [UInt64](../../sql-reference/data-types/int-uint.md)-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`.
## dictGetChildren
Returns first-level children as an array of indexes. It is the inverse transformation for [dictGetHierarchy](#dictgethierarchy).
**Syntax**
``` sql
dictGetChildren(dict_name, key)
```
**Arguments**
- `dict_name` — Name of the dictionary. [String literal](../../sql-reference/syntax.md#syntax-string-literal).
- `key` — Key value. [Expression](../../sql-reference/syntax.md#syntax-expressions) returning a [UInt64](../../sql-reference/data-types/int-uint.md)-type value.
**Returned values**
- First-level descendants for the key.
Type: [Array](../../sql-reference/data-types/array.md)([UInt64](../../sql-reference/data-types/int-uint.md)).
**Example**
Consider the hierarchic dictionary:
``` text
┌─id─┬─parent_id─┐
│ 1 │ 0 │
│ 2 │ 1 │
│ 3 │ 1 │
│ 4 │ 2 │
└────┴───────────┘
```
First-level children:
``` sql
SELECT dictGetChildren('hierarchy_flat_dictionary', number) FROM system.numbers LIMIT 4;
```
``` text
┌─dictGetChildren('hierarchy_flat_dictionary', number)─┐
│ [1] │
│ [2,3] │
│ [4] │
│ [] │
└──────────────────────────────────────────────────────┘
```
## dictGetDescendant
Returns all descendants as if [dictGetChildren](#dictgetchildren) function was applied `level` times recursively.
**Syntax**
``` sql
dictGetDescendants(dict_name, key, level)
```
**Arguments**
- `dict_name` — Name of the dictionary. [String literal](../../sql-reference/syntax.md#syntax-string-literal).
- `key` — Key value. [Expression](../../sql-reference/syntax.md#syntax-expressions) returning a [UInt64](../../sql-reference/data-types/int-uint.md)-type value.
- `level` — Hierarchy level. If `level = 0` returns all descendants to the end. [UInt8](../../sql-reference/data-types/int-uint.md).
**Returned values**
- Descendants for the key.
Type: [Array](../../sql-reference/data-types/array.md)([UInt64](../../sql-reference/data-types/int-uint.md)).
**Example**
Consider the hierarchic dictionary:
``` text
┌─id─┬─parent_id─┐
│ 1 │ 0 │
│ 2 │ 1 │
│ 3 │ 1 │
│ 4 │ 2 │
└────┴───────────┘
```
All descendants:
``` sql
SELECT dictGetDescendants('hierarchy_flat_dictionary', number) FROM system.numbers LIMIT 4;
```
``` text
┌─dictGetDescendants('hierarchy_flat_dictionary', number)─┐
│ [1,2,3,4] │
│ [2,3,4] │
│ [4] │
│ [] │
└─────────────────────────────────────────────────────────┘
```
First-level descendants:
``` sql
SELECT dictGetDescendants('hierarchy_flat_dictionary', number, 1) FROM system.numbers LIMIT 4;
```
``` text
┌─dictGetDescendants('hierarchy_flat_dictionary', number, 1)─┐
│ [1] │
│ [2,3] │
│ [4] │
│ [] │
└────────────────────────────────────────────────────────────┘
```
## 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:
``` sql
dictGet[Type]('dict_name', 'attr_name', id_expr)
dictGet[Type]OrDefault('dict_name', 'attr_name', id_expr, default_value_expr)
```
**Arguments**
- `dict_name` — Name of the dictionary. [String literal](../../sql-reference/syntax.md#syntax-string-literal).
- `attr_name` — Name of the column of the dictionary. [String literal](../../sql-reference/syntax.md#syntax-string-literal).
- `id_expr` — Key value. [Expression](../../sql-reference/syntax.md#syntax-expressions) returning a [UInt64](../../sql-reference/data-types/int-uint.md) or [Tuple](../../sql-reference/data-types/tuple.md)-type value depending on the dictionary configuration.
- `default_value_expr` — Value returned if the dictionary does not contain a row with the `id_expr` key. [Expression](../../sql-reference/syntax.md#syntax-expressions) 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](../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md#ext_dict_structure-attributes), 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 does not match the attribute data type.
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