Database Reference
In-Depth Information
RAND(UNIX_TIMESTAMP())
RAND(CONNECTION_ID())
RAND(UNIX_TIMESTAMP()+CONNECTION_ID())
However, it's probably better to use other seed value sources if you have them. For
example, if your system has a
/dev/random
or
/dev/urandom
device, read the device and
use it to generate a value for seeding
RAND()
.
How Random Is RAND()?
Does the
RAND()
function generate evenly distributed numbers? Check it out for yourself
with the following Python script,
rand_test.py
, from the
stats
directory of the
recipes
distribution. (That directory also contains equivalent scripts in other languages.) The
script uses
RAND()
to generate random numbers and constructs a frequency distribution
from them, using 10 categories (“buckets”). This provides a means of assessing how
evenly distributed the values are:
#!/usr/bin/python
# rand_test.pl: create a frequency distribution of RAND() values.
# This provides a test of the randomness of RAND().
# Method: Draw random numbers in the range from 0 to 1.0,
# and count how many of them occur in .1-sized intervals
import
cookbook
npicks
=
1000
# number of times to pick a number
bucket
=
[
0
]
*
10
# buckets for counting picks in each interval
conn
=
cookbook
.
connect
()
cursor
=
conn
.
cursor
()
for
i
in
range
(
0
,
npicks
):
cursor
.
execute
(
"SELECT RAND()"
)
(
val
,)
=
cursor
.
fetchone
()
slot
=
int
(
val
*
10
)
if
slot
>
9
:
slot
=
9
# put 1.0 in last slot
bucket
[
slot
]
+=
1
cursor
.
close
()
conn
.
close
()
# Print the resulting frequency distribution
for
slot
,
val
in
enumerate
(
bucket
):
print
(
"
%2d
%d
"
%
(
slot
+
1
,
val
))
