mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-11-28 02:21:59 +00:00
337 lines
5.7 KiB
Markdown
337 lines
5.7 KiB
Markdown
---
|
|
slug: /en/sql-reference/functions/functions-for-nulls
|
|
sidebar_position: 135
|
|
sidebar_label: Nullable
|
|
---
|
|
|
|
# Functions for Working with Nullable Values
|
|
|
|
## isNull
|
|
|
|
Returns whether the argument is [NULL](../../sql-reference/syntax.md#null).
|
|
|
|
``` sql
|
|
isNull(x)
|
|
```
|
|
|
|
Alias: `ISNULL`.
|
|
|
|
**Arguments**
|
|
|
|
- `x` — A value of non-compound data type.
|
|
|
|
**Returned value**
|
|
|
|
- `1` if `x` is `NULL`.
|
|
- `0` if `x` is not `NULL`.
|
|
|
|
**Example**
|
|
|
|
Table:
|
|
|
|
``` text
|
|
┌─x─┬────y─┐
|
|
│ 1 │ ᴺᵁᴸᴸ │
|
|
│ 2 │ 3 │
|
|
└───┴──────┘
|
|
```
|
|
|
|
Query:
|
|
|
|
``` sql
|
|
SELECT x FROM t_null WHERE isNull(y);
|
|
```
|
|
|
|
Result:
|
|
|
|
``` text
|
|
┌─x─┐
|
|
│ 1 │
|
|
└───┘
|
|
```
|
|
|
|
## isNotNull
|
|
|
|
Returns whether the argument is not [NULL](../../sql-reference/syntax.md#null-literal).
|
|
|
|
``` sql
|
|
isNotNull(x)
|
|
```
|
|
|
|
**Arguments:**
|
|
|
|
- `x` — A value of non-compound data type.
|
|
|
|
**Returned value**
|
|
|
|
- `1` if `x` is not `NULL`.
|
|
- `0` if `x` is `NULL`.
|
|
|
|
**Example**
|
|
|
|
Table:
|
|
|
|
``` text
|
|
┌─x─┬────y─┐
|
|
│ 1 │ ᴺᵁᴸᴸ │
|
|
│ 2 │ 3 │
|
|
└───┴──────┘
|
|
```
|
|
|
|
Query:
|
|
|
|
``` sql
|
|
SELECT x FROM t_null WHERE isNotNull(y);
|
|
```
|
|
|
|
Result:
|
|
|
|
``` text
|
|
┌─x─┐
|
|
│ 2 │
|
|
└───┘
|
|
```
|
|
|
|
## coalesce
|
|
|
|
Returns the leftmost non-`NULL` argument.
|
|
|
|
``` sql
|
|
coalesce(x,...)
|
|
```
|
|
|
|
**Arguments:**
|
|
|
|
- Any number of parameters of non-compound type. All parameters must be of mutually compatible data types.
|
|
|
|
**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─────┬──telegram─┐
|
|
│ 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 name, coalesce(mail, phone, CAST(telegram,'Nullable(String)')) FROM aBook;
|
|
```
|
|
|
|
``` text
|
|
┌─name─────┬─coalesce(mail, phone, CAST(telegram, 'Nullable(String)'))─┐
|
|
│ client 1 │ 123-45-67 │
|
|
│ client 2 │ ᴺᵁᴸᴸ │
|
|
└──────────┴───────────────────────────────────────────────────────────┘
|
|
```
|
|
|
|
## ifNull
|
|
|
|
Returns an alternative value if the 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**
|
|
|
|
- `x` if `x` is not `NULL`.
|
|
- `alt` if `x` is `NULL`.
|
|
|
|
**Example**
|
|
|
|
Query:
|
|
|
|
``` sql
|
|
SELECT ifNull('a', 'b');
|
|
```
|
|
|
|
Result:
|
|
|
|
``` text
|
|
┌─ifNull('a', 'b')─┐
|
|
│ a │
|
|
└──────────────────┘
|
|
```
|
|
|
|
Query:
|
|
|
|
``` sql
|
|
SELECT ifNull(NULL, 'b');
|
|
```
|
|
|
|
Result:
|
|
|
|
``` text
|
|
┌─ifNull(NULL, 'b')─┐
|
|
│ b │
|
|
└───────────────────┘
|
|
```
|
|
|
|
## nullIf
|
|
|
|
Returns `NULL` if both arguments are equal.
|
|
|
|
``` sql
|
|
nullIf(x, y)
|
|
```
|
|
|
|
**Arguments:**
|
|
|
|
`x`, `y` — Values to compare. Must be of compatible types.
|
|
|
|
**Returned values**
|
|
|
|
- `NULL` if the arguments are equal.
|
|
- `x` if the arguments are not equal.
|
|
|
|
**Example**
|
|
|
|
Query:
|
|
|
|
``` sql
|
|
SELECT nullIf(1, 1);
|
|
```
|
|
|
|
Result:
|
|
|
|
``` text
|
|
┌─nullIf(1, 1)─┐
|
|
│ ᴺᵁᴸᴸ │
|
|
└──────────────┘
|
|
```
|
|
|
|
Query:
|
|
|
|
``` sql
|
|
SELECT nullIf(1, 2);
|
|
```
|
|
|
|
Result:
|
|
|
|
``` text
|
|
┌─nullIf(1, 2)─┐
|
|
│ 1 │
|
|
└──────────────┘
|
|
```
|
|
|
|
## assumeNotNull
|
|
|
|
Returns the corresponding non-`Nullable` value for a value of [Nullable](../../sql-reference/data-types/nullable.md) type. If the original value is `NULL`, an arbitrary result can be returned. See also functions `ifNull` and `coalesce`.
|
|
|
|
``` sql
|
|
assumeNotNull(x)
|
|
```
|
|
|
|
**Arguments:**
|
|
|
|
- `x` — The original value.
|
|
|
|
**Returned values**
|
|
|
|
- The input value as non-`Nullable` type, if it is not `NULL`.
|
|
- An arbitrary value, if the input value is `NULL`.
|
|
|
|
**Example**
|
|
|
|
Table:
|
|
|
|
``` text
|
|
|
|
┌─x─┬────y─┐
|
|
│ 1 │ ᴺᵁᴸᴸ │
|
|
│ 2 │ 3 │
|
|
└───┴──────┘
|
|
```
|
|
|
|
Query:
|
|
|
|
``` sql
|
|
SELECT assumeNotNull(y) FROM table;
|
|
```
|
|
|
|
Result:
|
|
|
|
``` text
|
|
┌─assumeNotNull(y)─┐
|
|
│ 0 │
|
|
│ 3 │
|
|
└──────────────────┘
|
|
```
|
|
|
|
Query:
|
|
|
|
``` sql
|
|
SELECT toTypeName(assumeNotNull(y)) FROM t_null;
|
|
```
|
|
|
|
Result:
|
|
|
|
``` text
|
|
┌─toTypeName(assumeNotNull(y))─┐
|
|
│ Int8 │
|
|
│ Int8 │
|
|
└──────────────────────────────┘
|
|
```
|
|
|
|
## toNullable
|
|
|
|
Converts the argument type to `Nullable`.
|
|
|
|
``` sql
|
|
toNullable(x)
|
|
```
|
|
|
|
**Arguments:**
|
|
|
|
- `x` — A value of non-compound type.
|
|
|
|
**Returned value**
|
|
|
|
- The input value but of `Nullable` type.
|
|
|
|
**Example**
|
|
|
|
Query:
|
|
|
|
``` sql
|
|
SELECT toTypeName(10);
|
|
```
|
|
|
|
Result:
|
|
|
|
``` text
|
|
┌─toTypeName(10)─┐
|
|
│ UInt8 │
|
|
└────────────────┘
|
|
```
|
|
|
|
Query:
|
|
|
|
``` sql
|
|
SELECT toTypeName(toNullable(10));
|
|
```
|
|
|
|
Result:
|
|
|
|
``` text
|
|
┌─toTypeName(toNullable(10))─┐
|
|
│ Nullable(UInt8) │
|
|
└────────────────────────────┘
|
|
```
|