ClickHouse/docs/en/sql-reference/functions/functions-for-nulls.md
2021-05-03 13:39:10 +03:00

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---
toc_priority: 63
toc_title: Nullable
---
# Functions for Working with Nullable Values {#functions-for-working-with-nullable-aggregates}
## isNull {#isnull}
Checks whether the argument is [NULL](../../sql-reference/syntax.md#null-literal).
``` sql
isNull(x)
```
Alias: `ISNULL`.
**Arguments**
- `x` — A value with a non-compound data type.
**Returned value**
- `1` if `x` is `NULL`.
- `0` if `x` is not `NULL`.
**Example**
Input table
``` text
┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │ 3 │
└───┴──────┘
```
Query
``` sql
SELECT x FROM t_null WHERE isNull(y);
```
``` text
┌─x─┐
│ 1 │
└───┘
```
## isNotNull {#isnotnull}
Checks whether the argument is [NULL](../../sql-reference/syntax.md#null-literal).
``` sql
isNotNull(x)
```
**Arguments:**
- `x` — A value with a non-compound data type.
**Returned value**
- `0` if `x` is `NULL`.
- `1` if `x` is not `NULL`.
**Example**
Input table
``` text
┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │ 3 │
└───┴──────┘
```
Query
``` sql
SELECT x FROM t_null WHERE isNotNull(y);
```
``` text
┌─x─┐
│ 2 │
└───┘
```
## coalesce {#coalesce}
Checks from left to right whether `NULL` arguments were passed and returns the first non-`NULL` argument.
``` sql
coalesce(x,...)
```
**Arguments:**
- Any number of parameters of a non-compound type. All parameters must be compatible by data type.
**Returned values**
- The first non-`NULL` argument.
- `NULL`, if all arguments are `NULL`.
**Example**
Consider a list of contacts that may specify multiple ways to contact a customer.
``` text
┌─name─────┬─mail─┬─phone─────┬──icq─┐
│ client 1 │ ᴺᵁᴸᴸ │ 123-45-67 │ 123 │
│ client 2 │ ᴺᵁᴸᴸ │ ᴺᵁᴸᴸ │ ᴺᵁᴸᴸ │
└──────────┴──────┴───────────┴──────┘
```
The `mail` and `phone` fields are of type String, but the `icq` field is `UInt32`, so it needs to be converted to `String`.
Get the first available contact method for the customer from the contact list:
``` sql
SELECT coalesce(mail, phone, CAST(icq,'Nullable(String)')) FROM aBook;
```
``` text
┌─name─────┬─coalesce(mail, phone, CAST(icq, 'Nullable(String)'))─┐
│ client 1 │ 123-45-67 │
│ client 2 │ ᴺᵁᴸᴸ │
└──────────┴──────────────────────────────────────────────────────┘
```
## ifNull {#ifnull}
Returns an alternative value if the main argument is `NULL`.
``` sql
ifNull(x,alt)
```
**Arguments:**
- `x` — The value to check for `NULL`.
- `alt` — The value that the function returns if `x` is `NULL`.
**Returned values**
- The value `x`, if `x` is not `NULL`.
- The value `alt`, if `x` is `NULL`.
**Example**
``` sql
SELECT ifNull('a', 'b');
```
``` text
┌─ifNull('a', 'b')─┐
│ a │
└──────────────────┘
```
``` sql
SELECT ifNull(NULL, 'b');
```
``` text
┌─ifNull(NULL, 'b')─┐
│ b │
└───────────────────┘
```
## nullIf {#nullif}
Returns `NULL` if the arguments are equal.
``` sql
nullIf(x, y)
```
**Arguments:**
`x`, `y` — Values for comparison. They must be compatible types, or ClickHouse will generate an exception.
**Returned values**
- `NULL`, if the arguments are equal.
- The `x` value, if the arguments are not equal.
**Example**
``` sql
SELECT nullIf(1, 1);
```
``` text
┌─nullIf(1, 1)─┐
│ ᴺᵁᴸᴸ │
└──────────────┘
```
``` sql
SELECT nullIf(1, 2);
```
``` text
┌─nullIf(1, 2)─┐
│ 1 │
└──────────────┘
```
## assumeNotNull {#assumenotnull}
Results in a value of type [Nullable](../../sql-reference/data-types/nullable.md) for a non- `Nullable`, if the value is not `NULL`.
``` sql
assumeNotNull(x)
```
**Arguments:**
- `x` — The original value.
**Returned values**
- The original value from the non-`Nullable` type, if it is not `NULL`.
- Implementation specific result if the original value was `NULL`.
**Example**
Consider the `t_null` table.
``` sql
SHOW CREATE TABLE t_null;
```
``` text
┌─statement─────────────────────────────────────────────────────────────────┐
│ CREATE TABLE default.t_null ( x Int8, y Nullable(Int8)) ENGINE = TinyLog │
└───────────────────────────────────────────────────────────────────────────┘
```
``` text
┌─x─┬────y─┐
│ 1 │ ᴺᵁᴸᴸ │
│ 2 │ 3 │
└───┴──────┘
```
Apply the `assumeNotNull` function to the `y` column.
``` sql
SELECT assumeNotNull(y) FROM t_null;
```
``` text
┌─assumeNotNull(y)─┐
│ 0 │
│ 3 │
└──────────────────┘
```
``` sql
SELECT toTypeName(assumeNotNull(y)) FROM t_null;
```
``` text
┌─toTypeName(assumeNotNull(y))─┐
│ Int8 │
│ Int8 │
└──────────────────────────────┘
```
## toNullable {#tonullable}
Converts the argument type to `Nullable`.
``` sql
toNullable(x)
```
**Arguments:**
- `x` — The value of any non-compound type.
**Returned value**
- The input value with a `Nullable` type.
**Example**
``` sql
SELECT toTypeName(10);
```
``` text
┌─toTypeName(10)─┐
│ UInt8 │
└────────────────┘
```
``` sql
SELECT toTypeName(toNullable(10));
```
``` text
┌─toTypeName(toNullable(10))─┐
│ Nullable(UInt8) │
└────────────────────────────┘
```