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

---
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.