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297 lines
6.1 KiB
Markdown
297 lines
6.1 KiB
Markdown
# Functions for working with Nullable aggregates
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## isNull
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Checks whether the argument is [NULL](../syntax.md#null).
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```sql
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isNull(x)
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```
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**Parameters**
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- `x` — A value with a non-compound data type.
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**Returned value**
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- `1` if `x` is `NULL`.
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- `0` if `x` is not `NULL`.
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**Example**
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Input table
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```text
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┌─x─┬────y─┐
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│ 1 │ ᴺᵁᴸᴸ │
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│ 2 │ 3 │
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└───┴──────┘
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```
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Query
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```sql
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SELECT x FROM t_null WHERE isNull(y)
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```
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```text
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┌─x─┐
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│ 1 │
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└───┘
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```
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## isNotNull
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Checks whether the argument is [NULL](../syntax.md#null).
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```sql
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isNotNull(x)
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```
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**Parameters:**
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- `x` — A value with a non-compound data type.
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**Returned value**
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- `0` if `x` is `NULL`.
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- `1` if `x` is not `NULL`.
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**Example**
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Input table
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```text
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┌─x─┬────y─┐
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│ 1 │ ᴺᵁᴸᴸ │
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│ 2 │ 3 │
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└───┴──────┘
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```
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Query
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```sql
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SELECT x FROM t_null WHERE isNotNull(y)
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```
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```text
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┌─x─┐
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│ 2 │
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└───┘
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```
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## coalesce
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Checks from left to right whether `NULL` arguments were passed and returns the first non-`NULL` argument.
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```sql
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coalesce(x,...)
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```
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**Parameters:**
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- Any number of parameters of a non-compound type. All parameters must be compatible by data type.
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**Returned values**
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- The first non-`NULL` argument.
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- `NULL`, if all arguments are `NULL`.
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**Example**
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Consider a list of contacts that may specify multiple ways to contact a customer.
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```text
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┌─name─────┬─mail─┬─phone─────┬──icq─┐
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│ client 1 │ ᴺᵁᴸᴸ │ 123-45-67 │ 123 │
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│ client 2 │ ᴺᵁᴸᴸ │ ᴺᵁᴸᴸ │ ᴺᵁᴸᴸ │
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└──────────┴──────┴───────────┴──────┘
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```
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The `mail` and `phone` fields are of type String, but the `icq` field is `UInt32`, so it needs to be converted to `String`.
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Get the first available contact method for the customer from the contact list:
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```sql
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SELECT coalesce(mail, phone, CAST(icq,'Nullable(String)')) FROM aBook
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```
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```text
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┌─name─────┬─coalesce(mail, phone, CAST(icq, 'Nullable(String)'))─┐
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│ client 1 │ 123-45-67 │
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│ client 2 │ ᴺᵁᴸᴸ │
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└──────────┴──────────────────────────────────────────────────────┘
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```
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## ifNull
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Returns an alternative value if the main argument is `NULL`.
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```sql
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ifNull(x,alt)
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```
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**Parameters:**
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- `x` — The value to check for `NULL`.
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- `alt` — The value that the function returns if `x` is `NULL`.
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**Returned values**
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- The value `x`, if `x` is not `NULL`.
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- The value `alt`, if `x` is `NULL`.
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**Example**
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```sql
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SELECT ifNull('a', 'b')
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```
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```text
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┌─ifNull('a', 'b')─┐
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│ a │
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└──────────────────┘
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```
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```sql
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SELECT ifNull(NULL, 'b')
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```
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```text
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┌─ifNull(NULL, 'b')─┐
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│ b │
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└───────────────────┘
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```
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## nullIf
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Returns `NULL` if the arguments are equal.
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```sql
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nullIf(x, y)
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```
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**Parameters:**
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`x`, `y` — Values for comparison. They must be compatible types, or ClickHouse will generate an exception.
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**Returned values**
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- `NULL`, if the arguments are equal.
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- The `x` value, if the arguments are not equal.
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**Example**
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```sql
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SELECT nullIf(1, 1)
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```
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```text
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┌─nullIf(1, 1)─┐
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│ ᴺᵁᴸᴸ │
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└──────────────┘
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```
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```sql
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SELECT nullIf(1, 2)
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```
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```text
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┌─nullIf(1, 2)─┐
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│ 1 │
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└──────────────┘
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```
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## assumeNotNull
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Results in a value of type [Nullable](../../data_types/nullable.md) for a non- `Nullable`, if the value is not `NULL`.
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```sql
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assumeNotNull(x)
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```
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**Parameters:**
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- `x` — The original value.
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**Returned values**
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- The original value from the non-`Nullable` type, if it is not `NULL`.
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- The default value for the non-`Nullable` type if the original value was `NULL`.
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**Example**
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Consider the `t_null` table.
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```sql
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SHOW CREATE TABLE t_null
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```
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```text
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┌─statement─────────────────────────────────────────────────────────────────┐
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│ CREATE TABLE default.t_null ( x Int8, y Nullable(Int8)) ENGINE = TinyLog │
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└───────────────────────────────────────────────────────────────────────────┘
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```
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```text
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┌─x─┬────y─┐
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│ 1 │ ᴺᵁᴸᴸ │
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│ 2 │ 3 │
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└───┴──────┘
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```
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Apply the `assumeNotNull` function to the `y` column.
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```sql
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SELECT assumeNotNull(y) FROM t_null
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```
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```text
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┌─assumeNotNull(y)─┐
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│ 0 │
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│ 3 │
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└──────────────────┘
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```
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```sql
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SELECT toTypeName(assumeNotNull(y)) FROM t_null
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```
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```text
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┌─toTypeName(assumeNotNull(y))─┐
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│ Int8 │
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│ Int8 │
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└──────────────────────────────┘
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```
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## toNullable
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Converts the argument type to `Nullable`.
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```sql
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toNullable(x)
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```
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**Parameters:**
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- `x` — The value of any non-compound type.
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**Returned value**
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- The input value with a `Nullable` type.
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**Example**
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```sql
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SELECT toTypeName(10)
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```
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```text
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┌─toTypeName(10)─┐
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│ UInt8 │
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└────────────────┘
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```
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```sql
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SELECT toTypeName(toNullable(10))
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```
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```text
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┌─toTypeName(toNullable(10))─┐
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│ Nullable(UInt8) │
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└────────────────────────────┘
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```
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[Original article](https://clickhouse.yandex/docs/en/query_language/functions/functions_for_nulls/) <!--hide-->
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