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Data Generation Functions

NORMAL

Generates a normally-distributed pseudo-random floating point number with specified mean and stddev (standard deviation).

Syntax

NORMAL( <mean> , <stddev> , <gen> )
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Arguments

mean

A constant specifying the value that the output values should be centered on.

stddev

A constant specifying the width of one standard deviation.

For example, if you specify a mean of 0.0 and a standard deviation of 1.0, approximately 68.2% of returned values from multiple calls will be between -1.0 and +1.0 (i.e. within one standard deviation of the mean).

Similarly, if you choose a mean of 5.0 and a standard deviation of 2, then approximately 68.2% of values will be between 3.0 and 7.0.

gen

An expression that serves as a raw source of uniform random numbers, typically the RANDOM function. For more information, see the Data Generation Functions Usage Notes.

Returns

Returns a random floating-point number. The accumulated results of a large number of repeated calls approximate a normal distribution.

Usage notes

This function is related to, but different from, the RANDOM function, both in the ranges of the values returned and their distribution.

  • RANDOM generates random 64-bit integers in a uniform distribution. It accepts an optional seed that allows random sequences to be repeated.

    When RANDOM is called a large number of times, the results are more or less evenly distributed over the range of possible values. For example, the number of results with values between 1000 and 2000 is similar to the number of values between 2000 and 3000.

  • NORMAL generates random integer or floating-point numbers centered on the specified mean, with the specified standard deviation.

    When NORMAL is called a large number of times, the distribution of the results is likely to approximate a “normal” curve (a “bell-shaped curve”). The center of the curve and its “breadth” are influenced by the mean and stddev parameters. Values closer to the specified mean are more likely to occur than values far from the mean.

Examples

This shows typical usage with a mean of 0 and a standard deviation of 1:

SELECT normal(0, 1, random()) FROM table(generator(rowCount => 5));

+------------------------+
| NORMAL(0, 1, RANDOM()) |
|------------------------|
|           0.227384164  |
|           0.9945290748 |
|          -0.2045078571 |
|          -1.594607893  |
|          -0.8213296842 |
+------------------------+
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This shows that if the gen parameter is a constant, then the output is a constant:

SELECT normal(0, 1, 1234) FROM table(generator(rowCount => 5));

+--------------------+
| NORMAL(0, 1, 1234) |
|--------------------|
|      -0.6604156716 |
|      -0.6604156716 |
|      -0.6604156716 |
|      -0.6604156716 |
|      -0.6604156716 |
+--------------------+
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