ClickHouse/docs/en/sql-reference/functions/random-functions.md

---
slug: /en/sql-reference/functions/random-functions
sidebar_position: 145
sidebar_label: Random Numbers
---

# Functions for Generating Random Numbers

All functions in this section accept zero or one arguments. The only use of the argument (if provided) is to prevent prevent [common subexpression
elimination](../../sql-reference/functions/index.md#common-subexpression-elimination) such that two different execution of the same random
function in a query return different random values.

Related content
- Blog: [Generating random data in ClickHouse](https://clickhouse.com/blog/generating-random-test-distribution-data-for-clickhouse)

:::note
The random numbers are generated by non-cryptographic algorithms.
:::

## rand, rand32

Returns a random UInt32 number, evenly distributed across the range of all possible UInt32 numbers.

Uses a linear congruential generator.

## rand64

Returns a random UInt64 number, evenly distributed across the range of all possible UInt64 numbers.

Uses a linear congruential generator.

## randCanonical

Returns a random Float64 value, evenly distributed in interval [0, 1).

## randConstant

Like `rand` but produces a constant column with a random value.

**Example**

``` sql
SELECT rand(), rand(1), rand(number), randConstant(), randConstant(1), randConstant(number)
FROM numbers(3)
```

Result:

``` result
┌─────rand()─┬────rand(1)─┬─rand(number)─┬─randConstant()─┬─randConstant(1)─┬─randConstant(number)─┐
│ 3047369878 │ 4132449925 │   4044508545 │     2740811946 │      4229401477 │           1924032898 │
│ 2938880146 │ 1267722397 │   4154983056 │     2740811946 │      4229401477 │           1924032898 │
│  956619638 │ 4238287282 │   1104342490 │     2740811946 │      4229401477 │           1924032898 │
└────────────┴────────────┴──────────────┴────────────────┴─────────────────┴──────────────────────┘
```

## randUniform

Returns a random Float64 drawn uniformly from interval [`min`, `max`) ([continuous uniform distribution](https://en.wikipedia.org/wiki/Continuous_uniform_distribution)).

**Syntax**

``` sql
randUniform(min, max)
```

**Arguments**

- `min` - `Float64` - left boundary of the range,
- `max` - `Float64` - right boundary of the range.

**Returned value**

- Random number.

Type: [Float64](/docs/en/sql-reference/data-types/float.md).

**Example**

``` sql
SELECT randUniform(5.5, 10) FROM numbers(5)
```

Result:

``` result
┌─randUniform(5.5, 10)─┐
│    8.094978491443102 │
│   7.3181248914450885 │
│    7.177741903868262 │
│    6.483347380953762 │
│    6.122286382885112 │
└──────────────────────┘
```

## randNormal

Returns a random Float64 drawn from a [normal distribution](https://en.wikipedia.org/wiki/Normal_distribution).

**Syntax**

``` sql
randNormal(mean, variance)
```

**Arguments**

- `mean` - `Float64` - mean value of distribution,
- `variance` - `Float64` - [variance](https://en.wikipedia.org/wiki/Variance) of the distribution.

**Returned value**

- Random number.

Type: [Float64](/docs/en/sql-reference/data-types/float.md).

**Example**

``` sql
SELECT randNormal(10, 2) FROM numbers(5)
```

Result:

``` result
┌──randNormal(10, 2)─┐
│ 13.389228911709653 │
│  8.622949707401295 │
│ 10.801887062682981 │
│ 4.5220192605895315 │
│ 10.901239123982567 │
└────────────────────┘
```

## randLogNormal

Returns a random Float64 drawn from a [log-normal distribution](https://en.wikipedia.org/wiki/Log-normal_distribution).

**Syntax**

``` sql
randLogNormal(mean, variance)
```

**Arguments**

- `mean` - `Float64` - mean value of distribution,
- `variance` - `Float64` - [variance](https://en.wikipedia.org/wiki/Variance) of the distribution.

**Returned value**

- Random number.

Type: [Float64](/docs/en/sql-reference/data-types/float.md).

**Example**

``` sql
SELECT randLogNormal(100, 5) FROM numbers(5)
```

Result:

``` result
┌─randLogNormal(100, 5)─┐
│  1.295699673937363e48 │
│  9.719869109186684e39 │
│  6.110868203189557e42 │
│  9.912675872925529e39 │
│ 2.3564708490552458e42 │
└───────────────────────┘
```

## randBinomial

Returns a random UInt64 drawn from a [binomial distribution](https://en.wikipedia.org/wiki/Binomial_distribution).

**Syntax**

``` sql
randBinomial(experiments, probability)
```

**Arguments**

- `experiments` - `UInt64` - number of experiments,
- `probability` - `Float64` - probability of success in each experiment, a value between 0 and 1.

**Returned value**

- Random number.

Type: [UInt64](/docs/en/sql-reference/data-types/int-uint.md).

**Example**

``` sql
SELECT randBinomial(100, .75) FROM numbers(5)
```

Result:

``` result
┌─randBinomial(100, 0.75)─┐
│                      74 │
│                      78 │
│                      76 │
│                      77 │
│                      80 │
└─────────────────────────┘
```

## randNegativeBinomial

Returns a random UInt64 drawn from a [negative binomial distribution](https://en.wikipedia.org/wiki/Negative_binomial_distribution).

**Syntax**

``` sql
randNegativeBinomial(experiments, probability)
```

**Arguments**

- `experiments` - `UInt64` - number of experiments,
- `probability` - `Float64` - probability of failure in each experiment, a value between 0 and 1.

**Returned value**

- Random number.

Type: [UInt64](/docs/en/sql-reference/data-types/int-uint.md).

**Example**

``` sql
SELECT randNegativeBinomial(100, .75) FROM numbers(5)
```

Result:

``` result
┌─randNegativeBinomial(100, 0.75)─┐
│                              33 │
│                              32 │
│                              39 │
│                              40 │
│                              50 │
└─────────────────────────────────┘
```

## randPoisson

Returns a random UInt64 drawn from a [Poisson distribution](https://en.wikipedia.org/wiki/Poisson_distribution).

**Syntax**

``` sql
randPoisson(n)
```

**Arguments**

- `n` - `UInt64` - mean number of occurrences.

**Returned value**

- Random number.

Type: [UInt64](/docs/en/sql-reference/data-types/int-uint.md).

**Example**

``` sql
SELECT randPoisson(10) FROM numbers(5)
```

Result:

``` result
┌─randPoisson(10)─┐
│               8 │
│               8 │
│               7 │
│              10 │
│               6 │
└─────────────────┘
```

## randBernoulli

Returns a random UInt64 drawn from a [Bernoulli distribution](https://en.wikipedia.org/wiki/Bernoulli_distribution).

**Syntax**

``` sql
randBernoulli(probability)
```

**Arguments**

- `probability` - `Float64` - probability of success, a value between 0 and 1.

**Returned value**

- Random number.

Type: [UInt64](/docs/en/sql-reference/data-types/int-uint.md).

**Example**

``` sql
SELECT randBernoulli(.75) FROM numbers(5)
```

Result:

``` result
┌─randBernoulli(0.75)─┐
│                   1 │
│                   1 │
│                   0 │
│                   1 │
│                   1 │
└─────────────────────┘
```

## randExponential

Returns a random Float64 drawn from a [exponential distribution](https://en.wikipedia.org/wiki/Exponential_distribution).

**Syntax**

``` sql
randExponential(lambda)
```

**Arguments**

- `lambda` - `Float64` - lambda value.

**Returned value**

- Random number.

Type: [Float64](/docs/en/sql-reference/data-types/float.md).

**Example**

``` sql
SELECT randExponential(1/10) FROM numbers(5)
```

Result:

``` result
┌─randExponential(divide(1, 10))─┐
│              44.71628934340778 │
│              4.211013337903262 │
│             10.809402553207766 │
│              15.63959406553284 │
│             1.8148392319860158 │
└────────────────────────────────┘
```

## randChiSquared

Returns a random Float64 drawn from a [Chi-square distribution](https://en.wikipedia.org/wiki/Chi-squared_distribution) - a distribution of a sum of the squares of k independent standard normal random variables.

**Syntax**

``` sql
randChiSquared(degree_of_freedom)
```

**Arguments**

- `degree_of_freedom` - `Float64` - degree of freedom.

**Returned value**

- Random number.

Type: [Float64](/docs/en/sql-reference/data-types/float.md).

**Example**

``` sql
SELECT randChiSquared(10) FROM numbers(5)
```

Result:

``` result
┌─randChiSquared(10)─┐
│ 10.015463656521543 │
│  9.621799919882768 │
│   2.71785015634699 │
│ 11.128188665931908 │
│  4.902063104425469 │
└────────────────────┘
```

## randStudentT

Returns a random Float64 drawn from a [Student's t-distribution](https://en.wikipedia.org/wiki/Student%27s_t-distribution).

**Syntax**

``` sql
randStudentT(degree_of_freedom)
```

**Arguments**

- `degree_of_freedom` - `Float64` - degree of freedom.

**Returned value**

- Random number.

Type: [Float64](/docs/en/sql-reference/data-types/float.md).

**Example**

``` sql
SELECT randStudentT(10) FROM numbers(5)
```

Result:

``` result
┌─────randStudentT(10)─┐
│   1.2217309938538725 │
│   1.7941971681200541 │
│ -0.28192176076784664 │
│   0.2508897721303792 │
│  -2.7858432909761186 │
└──────────────────────┘
```

## randFisherF

Returns a random Float64 drawn from a [F-distribution](https://en.wikipedia.org/wiki/F-distribution).

**Syntax**

``` sql
randFisherF(d1, d2)
```

**Arguments**

- `d1` - `Float64` - d1 degree of freedom in `X = (S1 / d1) / (S2 / d2)`,
- `d2` - `Float64` - d2 degree of freedom in `X = (S1 / d1) / (S2 / d2)`,

**Returned value**

- Random number.

Type: [Float64](/docs/en/sql-reference/data-types/float.md).

**Example**

``` sql
SELECT randFisherF(10, 3) FROM numbers(5)
```

Result:

``` result
┌──randFisherF(10, 3)─┐
│   7.286287504216609 │
│ 0.26590779413050386 │
│ 0.22207610901168987 │
│  0.7953362728449572 │
│ 0.19278885985221572 │
└─────────────────────┘
```

## randomString {#randomString}

Generates a string of the specified length filled with random bytes (including zero bytes). Not all characters may be printable.

**Syntax**

``` sql
randomString(length)
```

**Arguments**

- `length` — String length in bytes. Positive integer.

**Returned value**

- String filled with random bytes.

Type: [String](../../sql-reference/data-types/string.md).

**Example**

Query:

``` sql
SELECT randomString(30) AS str, length(str) AS len FROM numbers(2) FORMAT Vertical;
```

Result:

``` text
Row 1:
──────
str: 3 G  :   pT ?w тi  k aV f6
len: 30

Row 2:
──────
str: 9 ,]    ^   )  ]??  8
len: 30
```

## randomFixedString

Generates a binary string of the specified length filled with random bytes (including zero bytes). Not all characters may be printable.

**Syntax**

``` sql
randomFixedString(length);
```

**Arguments**

- `length` — String length in bytes. [UInt64](../../sql-reference/data-types/int-uint.md).

**Returned value(s)**

- String filled with random bytes.

Type: [FixedString](../../sql-reference/data-types/fixedstring.md).

**Example**

Query:

```sql
SELECT randomFixedString(13) as rnd, toTypeName(rnd)
```

Result:

```text
┌─rnd──────┬─toTypeName(randomFixedString(13))─┐
│ j▒h㋖HɨZ'▒ │ FixedString(13)                 │
└──────────┴───────────────────────────────────┘
```

## randomPrintableASCII

Generates a string with a random set of [ASCII](https://en.wikipedia.org/wiki/ASCII#Printable_characters) characters. All characters are printable.
If you pass `length < 0`, the behavior of the function is undefined.

**Syntax**

``` sql
randomPrintableASCII(length)
```

**Arguments**

- `length` — String length in bytes. Positive integer.

**Returned value**

- String with a random set of [ASCII](https://en.wikipedia.org/wiki/ASCII#Printable_characters) printable characters.

Type: [String](../../sql-reference/data-types/string.md)

**Example**

``` sql
SELECT number, randomPrintableASCII(30) as str, length(str) FROM system.numbers LIMIT 3
```

``` text
┌─number─┬─str────────────────────────────┬─length(randomPrintableASCII(30))─┐
│      0 │ SuiCOSTvC0csfABSw=UcSzp2.`rv8x │                               30 │
│      1 │ 1Ag NlJ &RCN:*>HVPG;PE-nO"SUFD │                               30 │
│      2 │ /"+<"wUTh:=LjJ Vm!c&hI*m#XTfzz │                               30 │
└────────┴────────────────────────────────┴──────────────────────────────────┘
```

## randomStringUTF8

Generates a random string of a specified length. Result string contains valid UTF-8 code points. The value of code points may be outside of the range of assigned Unicode.

**Syntax**

``` sql
randomStringUTF8(length);
```

**Arguments**

- `length` — Length of the string in code points. [UInt64](../../sql-reference/data-types/int-uint.md).

**Returned value(s)**

- UTF-8 random string.

Type: [String](../../sql-reference/data-types/string.md).

**Example**

Query:

```sql
SELECT randomStringUTF8(13)
```

Result:

```text
┌─randomStringUTF8(13)─┐
│ 𘤗𙉝д兠庇󡅴󱱎󦐪􂕌𔊹𓰛   │
└──────────────────────┘
```

## fuzzBits {#fuzzBits}

**Syntax**

Flips the bits of String or FixedString `s`, each with probability `prob`.

**Syntax**

``` sql
fuzzBits(s, prob)
```

**Arguments**
- `s` - `String` or `FixedString`,
- `prob` - constant `Float32/64` between 0.0 and 1.0.

**Returned value**

Fuzzed string with same type as `s`.

**Example**

``` sql
SELECT fuzzBits(materialize('abacaba'), 0.1)
FROM numbers(3)
```

Result:

``` result
┌─fuzzBits(materialize('abacaba'), 0.1)─┐
│ abaaaja                               │
│ a*cjab+                               │
│ aeca2A                                │
└───────────────────────────────────────┘
```