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

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51	Pseudo-Random Numbers

Functions for Generating Pseudo-Random Numbers

All the functions accept zero arguments or one argument. If an argument is passed, it can be any type, and its value is not used for anything. The only purpose of this argument is to prevent common subexpression elimination, so that two different instances of the same function return different columns with different random numbers.

!!! note "Note" Non-cryptographic generators of pseudo-random numbers are used.

rand, rand32

Returns a pseudo-random UInt32 number, evenly distributed among all UInt32-type numbers.

Uses a linear congruential generator.

rand64

Returns a pseudo-random UInt64 number, evenly distributed among all UInt64-type numbers.

Uses a linear congruential generator.

randConstant

Produces a constant column with a random value.

Syntax

randConstant([x])

Arguments

• x — Expression resulting in any of the supported data types. The resulting value is discarded, but the expression itself if used for bypassing common subexpression elimination if the function is called multiple times in one query. Optional parameter.

Returned value

• Pseudo-random number.

Type: UInt32.

Example

Query:

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 │
└────────────┴────────────┴──────────────┴────────────────┴─────────────────┴──────────────────────┘

Random Functions for Working with Strings

randomString

randomFixedString

randomPrintableASCII

randomStringUTF8

fuzzBits

Syntax

fuzzBits([s], [prob])

Inverts bits of s, each with probability prob.

Arguments

• s - String or FixedString
• prob - constant Float32/64

Returned value Fuzzed string with same as s type.

Example

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

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