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 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

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

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

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).

Syntax

randUniform(min, max)

Arguments

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

Returned value

• Random number.

Type: Float64.

Example

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

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.

Syntax

randNormal(mean, variance)

Arguments

• mean - Float64 - mean value of distribution,
• variance - Float64 - variance of the distribution.

Returned value

• Random number.

Type: Float64.

Example

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

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.

Syntax

randLogNormal(mean, variance)

Arguments

• mean - Float64 - mean value of distribution,
• variance - Float64 - variance of the distribution.

Returned value

• Random number.

Type: Float64.

Example

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

Result:

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

randBinomial

Returns a random UInt64 drawn from a binomial distribution.

Syntax

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.

Example

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

Result:

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

randNegativeBinomial

Returns a random UInt64 drawn from a negative binomial distribution.

Syntax

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.

Example

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

Result:

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

randPoisson

Returns a random UInt64 drawn from a Poisson distribution.

Syntax

randPoisson(n)

Arguments

• n - UInt64 - mean number of occurrences.

Returned value

• Random number.

Type: UInt64.

Example

SELECT randPoisson(10) FROM numbers(5)

Result:

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

randBernoulli

Returns a random UInt64 drawn from a Bernoulli distribution.

Syntax

randBernoulli(probability)

Arguments

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

Returned value

• Random number.

Type: UInt64.

Example

SELECT randBernoulli(.75) FROM numbers(5)

Result:

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

randExponential

Returns a random Float64 drawn from a exponential distribution.

Syntax

randExponential(lambda)

Arguments

• lambda - Float64 - lambda value.

Returned value

• Random number.

Type: Float64.

Example

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

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 - a distribution of a sum of the squares of k independent standard normal random variables.

Syntax

randChiSquared(degree_of_freedom)

Arguments

• degree_of_freedom - Float64 - degree of freedom.

Returned value

• Random number.

Type: Float64.

Example

SELECT randChiSquared(10) FROM numbers(5)

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.

Syntax

randStudentT(degree_of_freedom)

Arguments

• degree_of_freedom - Float64 - degree of freedom.

Returned value

• Random number.

Type: Float64.

Example

SELECT randStudentT(10) FROM numbers(5)

Result:

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

randFisherF

Returns a random Float64 drawn from a F-distribution.

Syntax

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.

Example

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

Result:

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

randomString

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

Syntax

randomString(length)

Arguments

• length — String length in bytes. Positive integer.

Returned value

• String filled with random bytes.

Type: String.

Example

Query:

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

Result:

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

randomFixedString(length);

Arguments

• length — String length in bytes. UInt64.

Returned value(s)

• String filled with random bytes.

Type: FixedString.

Example

Query:

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

Result:

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

randomPrintableASCII

Generates a string with a random set of ASCII characters. All characters are printable. If you pass length < 0, the behavior of the function is undefined.

Syntax

randomPrintableASCII(length)

Arguments

• length — String length in bytes. Positive integer.

Returned value

• String with a random set of ASCII printable characters.

Type: String

Example

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

┌─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

randomStringUTF8(length);

Arguments

• length — Length of the string in code points. UInt64.

Returned value(s)

• UTF-8 random string.

Type: String.

Example

Query:

SELECT randomStringUTF8(13)

Result:

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

fuzzBits

Syntax

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

Syntax

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

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

Result:

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