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456 lines
8.3 KiB
Markdown
456 lines
8.3 KiB
Markdown
---
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slug: /en/sql-reference/functions/functions-for-nulls
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sidebar_position: 135
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sidebar_label: Nullable
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---
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# Functions for Working with Nullable Values
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## isNull
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Returns whether the argument is [NULL](../../sql-reference/syntax.md#null).
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See also operator [`IS NULL`](../operators/index.md#is_null).
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**Syntax**
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``` sql
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isNull(x)
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```
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Alias: `ISNULL`.
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**Arguments**
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- `x` — A value of 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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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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Result:
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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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## isNullable
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Returns `1` if a column is [Nullable](../data-types/nullable.md) (i.e allows `NULL` values), `0` otherwise.
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**Syntax**
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``` sql
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isNullable(x)
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```
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**Arguments**
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- `x` — column.
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**Returned value**
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- `1` if `x` allows `NULL` values. [UInt8](../data-types/int-uint.md).
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- `0` if `x` does not allow `NULL` values. [UInt8](../data-types/int-uint.md).
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**Example**
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Query:
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``` sql
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CREATE TABLE tab (ordinary_col UInt32, nullable_col Nullable(UInt32)) ENGINE = Log;
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INSERT INTO tab (ordinary_col, nullable_col) VALUES (1,1), (2, 2), (3,3);
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SELECT isNullable(ordinary_col), isNullable(nullable_col) FROM tab;
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```
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Result:
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``` text
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┌───isNullable(ordinary_col)──┬───isNullable(nullable_col)──┐
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1. │ 0 │ 1 │
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2. │ 0 │ 1 │
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3. │ 0 │ 1 │
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└─────────────────────────────┴─────────────────────────────┘
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```
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## isNotNull
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Returns whether the argument is not [NULL](../../sql-reference/syntax.md#null-literal).
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See also operator [`IS NOT NULL`](../operators/index.md#is_not_null).
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``` sql
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isNotNull(x)
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```
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**Arguments:**
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- `x` — A value of non-compound data type.
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**Returned value**
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- `1` if `x` is not `NULL`.
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- `0` if `x` is `NULL`.
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**Example**
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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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Result:
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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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## isNotDistinctFrom
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Performs null-safe comparison. Used to compare JOIN keys which contain NULL values in the JOIN ON section.
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This function will consider two `NULL` values as identical and will return `true`, which is distinct from the usual
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equals behavior where comparing two `NULL` values would return `NULL`.
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:::note
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This function is an internal function used by the implementation of JOIN ON. Please do not use it manually in queries.
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:::
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**Syntax**
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``` sql
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isNotDistinctFrom(x, y)
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```
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**Arguments**
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- `x` — first JOIN key.
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- `y` — second JOIN key.
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**Returned value**
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- `true` when `x` and `y` are both `NULL`.
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- `false` otherwise.
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**Example**
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For a complete example see: [NULL values in JOIN keys](../../sql-reference/statements/select/join#null-values-in-join-keys).
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## isZeroOrNull
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Returns whether the argument is 0 (zero) or [NULL](../../sql-reference/syntax.md#null-literal).
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``` sql
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isZeroOrNull(x)
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```
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**Arguments:**
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- `x` — A value of non-compound data type.
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**Returned value**
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- `1` if `x` is 0 (zero) or `NULL`.
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- `0` else.
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**Example**
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Table:
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``` text
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┌─x─┬────y─┐
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│ 1 │ ᴺᵁᴸᴸ │
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│ 2 │ 0 │
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│ 3 │ 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 isZeroOrNull(y);
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```
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Result:
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``` text
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┌─x─┐
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│ 1 │
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│ 2 │
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└───┘
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```
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## coalesce
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Returns the leftmost non-`NULL` argument.
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``` sql
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coalesce(x,...)
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```
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**Arguments:**
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- Any number of parameters of non-compound type. All parameters must be of mutually compatible data types.
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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─────┬──telegram─┐
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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 `telegram` 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 name, coalesce(mail, phone, CAST(telegram,'Nullable(String)')) FROM aBook;
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```
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``` text
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┌─name─────┬─coalesce(mail, phone, CAST(telegram, '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 argument is `NULL`.
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``` sql
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ifNull(x, alt)
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```
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**Arguments:**
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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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- `x` if `x` is not `NULL`.
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- `alt` if `x` is `NULL`.
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**Example**
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Query:
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``` sql
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SELECT ifNull('a', 'b');
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```
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Result:
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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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Query:
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``` sql
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SELECT ifNull(NULL, 'b');
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```
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Result:
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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 both arguments are equal.
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``` sql
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nullIf(x, y)
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```
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**Arguments:**
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`x`, `y` — Values to compare. Must be of compatible types.
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**Returned values**
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- `NULL` if the arguments are equal.
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- `x` if the arguments are not equal.
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**Example**
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Query:
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``` sql
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SELECT nullIf(1, 1);
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```
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Result:
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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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Query:
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``` sql
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SELECT nullIf(1, 2);
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```
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Result:
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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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Returns the corresponding non-`Nullable` value for a value of [Nullable](../data-types/nullable.md) type. If the original value is `NULL`, an arbitrary result can be returned. See also functions `ifNull` and `coalesce`.
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``` sql
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assumeNotNull(x)
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```
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**Arguments:**
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- `x` — The original value.
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**Returned values**
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- The input value as non-`Nullable` type, if it is not `NULL`.
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- An arbitrary value, if the input value is `NULL`.
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**Example**
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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 assumeNotNull(y) FROM table;
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```
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Result:
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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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Query:
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``` sql
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SELECT toTypeName(assumeNotNull(y)) FROM t_null;
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```
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Result:
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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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**Arguments:**
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- `x` — A value of non-compound type.
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**Returned value**
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- The input value but of `Nullable` type.
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**Example**
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Query:
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``` sql
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SELECT toTypeName(10);
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```
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Result:
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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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Query:
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``` sql
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SELECT toTypeName(toNullable(10));
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```
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Result:
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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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