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11.5.2. Mathematical Functions

Table 11.11. Mathematical Functions

Name Description
ABS() Return the absolute value
ACOS() Return the arc cosine
ASIN() Return the arc sine
ATAN2(), ATAN() Return the arc tangent of the two arguments
ATAN() Return the arc tangent
CEIL() Return the smallest integer value not less than the argument
CEILING() Return the smallest integer value not less than the argument
CONV() Convert numbers between different number bases
COS() Return the cosine
COT() Return the cotangent
CRC32()(v4.1.0) Compute a cyclic redundancy check value
DEGREES() Convert radians to degrees
EXP() Raise to the power of
FLOOR() Return the largest integer value not greater than the argument
LN() Return the natural logarithm of the argument
LOG10() Return the base-10 logarithm of the argument
LOG2() Return the base-2 logarithm of the argument
LOG() Return the natural logarithm of the first argument
MOD() Return the remainder
OCT() Return an octal representation of a decimal number
PI() Return the value of pi
POW() Return the argument raised to the specified power
POWER() Return the argument raised to the specified power
RADIANS() Return argument converted to radians
RAND() Return a random floating-point value
ROUND() Round the argument
SIGN() Return the sign of the argument
SIN() Return the sine of the argument
SQRT() Return the square root of the argument
TAN() Return the tangent of the argument
TRUNCATE() Truncate to specified number of decimal places

All mathematical functions return NULL in the event of an error.

  • ABS(X)

    Returns the absolute value of X. 

    mysql> SELECT ABS(2);
        -> 2
mysql> SELECT ABS(-32);
        -> 32

    This function is safe to use with BIGINT values.

  • ACOS(X)

    Returns the arc cosine of X, that is, the value whose cosine is X. Returns NULL if X is not in the range -1 to 1. 

    mysql> SELECT ACOS(1);
        -> 0
mysql> SELECT ACOS(1.0001);
        -> NULL
mysql> SELECT ACOS(0);
        -> 1.5707963267949

  • ASIN(X)

    Returns the arc sine of X, that is, the value whose sine is X. Returns NULL if X is not in the range -1 to 1. 

    mysql> SELECT ASIN(0.2);
        -> 0.20135792079033
mysql> SELECT ASIN('foo');

+-------------+
| ASIN('foo') |
+-------------+
|           0 |
+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> SHOW WARNINGS;
+---------+------+-----------------------------------------+
| Level   | Code | Message                                 |
+---------+------+-----------------------------------------+
| Warning | 1292 | Truncated incorrect DOUBLE value: 'foo' |
+---------+------+-----------------------------------------+

  • ATAN(X)

    Returns the arc tangent of X, that is, the value whose tangent is X. 

    mysql> SELECT ATAN(2);
        -> 1.1071487177941
mysql> SELECT ATAN(-2);
        -> -1.1071487177941

  • ATAN(Y,X), ATAN2(Y,X)

    Returns the arc tangent of the two variables X and Y. It is similar to calculating the arc tangent of Y / X, except that the signs of both arguments are used to determine the quadrant of the result. 

    mysql> SELECT ATAN(-2,2);
        -> -0.78539816339745
mysql> SELECT ATAN2(PI(),0);
        -> 1.5707963267949

  • CEIL(X)

    CEIL() is a synonym for CEILING().

  • CEILING(X)

    Returns the smallest integer value not less than X. 

    mysql> SELECT CEILING(1.23);
        -> 2
mysql> SELECT CEILING(-1.23);
        -> -1

    For exact-value numeric arguments, the return value has an exact-value numeric type. For string or floating-point arguments, the return value has a floating-point type.

  • CONV(N,from_base,to_base)

    Converts numbers between different number bases. Returns a string representation of the number N, converted from base from_base to base to_base. Returns NULL if any argument is NULL. The argument N is interpreted as an integer, but may be specified as an integer or a string. The minimum base is 2 and the maximum base is 36. If to_base is a negative number, N is regarded as a signed number. Otherwise, N is treated as unsigned. CONV() works with 64-bit precision. 

    mysql> SELECT CONV('a',16,2);
        -> '1010'
mysql> SELECT CONV('6E',18,8);
        -> '172'
mysql> SELECT CONV(-17,10,-18);
        -> '-H'
mysql> SELECT CONV(10+'10'+'10'+0xa,10,10);
        -> '40'

  • COS(X)

    Returns the cosine of X, where X is given in radians. 

    mysql> SELECT COS(PI());
        -> -1

  • COT(X)

    Returns the cotangent of X. 

    mysql> SELECT COT(12);
        -> -1.5726734063977
mysql> SELECT COT(0);
        -> NULL

  • CRC32(expr)

    Computes a cyclic redundancy check value and returns a 32-bit unsigned value. The result is NULL if the argument is NULL. The argument is expected to be a string and (if possible) is treated as one if it is not. 

    mysql> SELECT CRC32('MySQL');
        -> 3259397556
mysql> SELECT CRC32('mysql');
        -> 2501908538

  • DEGREES(X)

    Returns the argument X, converted from radians to degrees. 

    mysql> SELECT DEGREES(PI());
        -> 180
mysql> SELECT DEGREES(PI() / 2);
        -> 90

  • EXP(X)

    Returns the value of e (the base of natural logarithms) raised to the power of X. The inverse of this function is LOG() (using a single argumentonly) or LN(). 

    mysql> SELECT EXP(2);
        -> 7.3890560989307
mysql> SELECT EXP(-2);
        -> 0.13533528323661
mysql> SELECT EXP(0);
        -> 1

  • FLOOR(X)

    Returns the largest integer value not greater than X. 

    mysql> SELECT FLOOR(1.23);
        -> 1
mysql> SELECT FLOOR(-1.23);
        -> -2

    For exact-value numeric arguments, the return value has an exact-value numeric type. For string or floating-point arguments, the return value has a floating-point type.

  • FORMAT(X,D)

    Formats the number X to a format like '#,###,###.##', rounded to D decimal places, and returns the result as a string. For details, see Section 11.4, “String Functions”.

  • HEX(N_or_S)

    This function can be used to obtain a hexadecimal representation of a decimal number or a string; the manner in which it does so varies according to the argument's type. See this function's description in Section 11.4, “String Functions”, for details.

  • LN(X)

    Returns the natural logarithm of X; that is, the base-e logarithm of X. If X is less than or equal to 0, then NULL is returned. 

    mysql> SELECT LN(2);
        -> 0.69314718055995
mysql> SELECT LN(-2);
        -> NULL

    This function is synonymous with LOG(X). The inverse of this function is the EXP() function.

  • LOG(X), LOG(B,X)

    If called with one parameter, this function returns the natural logarithm of X. If X is less than or equal to 0, then NULL is returned.

    The inverse of this function (when called with a single argument) is the EXP() function. 

    mysql> SELECT LOG(2);
        -> 0.69314718055995
mysql> SELECT LOG(-2);
        -> NULL

    If called with two parameters, this function returns the logarithm of X to the base B. If X is less than or equal to 0, or if B is less than or equal to 1, then NULL is returned. 

    mysql> SELECT LOG(2,65536);
        -> 16
mysql> SELECT LOG(10,100);
        -> 2
mysql> SELECT LOG(1,100);
        -> NULL

    LOG(B,X) is equivalent to LOG(X) / LOG(B).

  • LOG2(X)

    Returns the base-2 logarithm of X. 

    mysql> SELECT LOG2(65536);
        -> 16
mysql> SELECT LOG2(-100);
        -> NULL

    LOG2() is useful for finding out how many bits a number requires for storage. This function is equivalent to the expression LOG(X) / LOG(2).

  • LOG10(X)

    Returns the base-10 logarithm of X. 

    mysql> SELECT LOG10(2);
        -> 0.30102999566398
mysql> SELECT LOG10(100);
        -> 2
mysql> SELECT LOG10(-100);
        -> NULL

    LOG10(X) is equivalent to LOG(10,X).

  • MOD(N,M), N % M, N MOD M

    Modulo operation. Returns the remainder of N divided by M. 

    mysql> SELECT MOD(234, 10);
        -> 4
mysql> SELECT 253 % 7;
        -> 1
mysql> SELECT MOD(29,9);
        -> 2
mysql> SELECT 29 MOD 9;
        -> 2

    This function is safe to use with BIGINT values.

    MOD() also works on values that have a fractional part and returns the exact remainder after division: 

    mysql> SELECT MOD(34.5,3);
        -> 1.5

    MOD(N,0) returns NULL.

  • PI()

    Returns the value of π (pi). The default number of decimal places displayed is seven, but MySQL uses the full double-precision value internally. 

    mysql> SELECT PI();
        -> 3.141593
mysql> SELECT PI()+0.000000000000000000;
        -> 3.141592653589793116

  • POW(X,Y)

    Returns the value of X raised to the power of Y. 

    mysql> SELECT POW(2,2);
        -> 4
mysql> SELECT POW(2,-2);
        -> 0.25

  • POWER(X,Y)

    This is a synonym for POW().

  • RADIANS(X)

    Returns the argument X, converted from degrees to radians. (Note that π radians equals 180 degrees.) 

    mysql> SELECT RADIANS(90);
        -> 1.5707963267949

  • RAND(), RAND(N)

    Returns a random floating-point value v in the range 0 <= v < 1.0. If a constant integer argument N is specified, it is used as the seed value, which produces a repeatable sequence of column values. In the following example, note that the sequences of values produced by RAND(3) is the same both places where it occurs. 

    mysql> CREATE TABLE t (i INT);
Query OK, 0 rows affected (0.42 sec)

mysql> INSERT INTO t VALUES(1),(2),(3);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT i, RAND() FROM t;
+------+------------------+
| i    | RAND()           |
+------+------------------+
|    1 | 0.61914388706828 |
|    2 | 0.93845168309142 |
|    3 | 0.83482678498591 |
+------+------------------+
3 rows in set (0.00 sec)

mysql> SELECT i, RAND(3) FROM t;
+------+------------------+
| i    | RAND(3)          |
+------+------------------+
|    1 | 0.90576975597606 |
|    2 | 0.37307905813035 |
|    3 | 0.14808605345719 |
+------+------------------+
3 rows in set (0.00 sec)

mysql> SELECT i, RAND() FROM t;
+------+------------------+
| i    | RAND()           |
+------+------------------+
|    1 | 0.35877890638893 |
|    2 | 0.28941420772058 |
|    3 | 0.37073435016976 |
+------+------------------+
3 rows in set (0.00 sec)

mysql> SELECT i, RAND(3) FROM t;
+------+------------------+
| i    | RAND(3)          |
+------+------------------+
|    1 | 0.90576975597606 |
|    2 | 0.37307905813035 |
|    3 | 0.14808605345719 |
+------+------------------+
3 rows in set (0.01 sec)

    The effect of using a nonconstant argument is undefined. As of MySQL 5.0.13, nonconstant arguments are disallowed.

    To obtain a random integer R in the range i <= R < j, use the expression FLOOR(i + RAND() * (j – i)). For example, to obtain a random integer in the range the range 7 <= R < 12, you could use the following statement: 

    SELECT FLOOR(7 + (RAND() * 5));

    RAND() in a WHERE clause is re-evaluated every time the WHERE is executed.

    You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times. However, you can retrieve rows in random order like this: 

    mysql> SELECT * FROM tbl_name ORDER BY RAND();

    ORDER BY RAND() combined with LIMIT is useful for selecting a random sample from a set of rows: 

    mysql> SELECT * FROM table1, table2 WHERE a=b AND c<d -> ORDER BY RAND() LIMIT 1000;

    RAND() is not meant to be a perfect random generator, but instead is a fast way to generate ad hoc random numbers which is portable between platforms for the same MySQL version.

  • ROUND(X), ROUND(X,D)

    Rounds the argument X to D decimal places. The rounding algorithm depends on the data type of X. D defaults to 0 if not specified. D can be negative to cause D digits left of the decimal point of the value X to become zero. 

    mysql> SELECT ROUND(-1.23);
        -> -1
mysql> SELECT ROUND(-1.58);
        -> -2
mysql> SELECT ROUND(1.58);
        -> 2
mysql> SELECT ROUND(1.298, 1);
        -> 1.3
mysql> SELECT ROUND(1.298, 0);
        -> 1
mysql> SELECT ROUND(23.298, -1);
        -> 20

    The return type is the same type as that of the first argument (assuming that it is integer, double, or decimal). This means that for an integer argument, the result is an integer (no decimal places): 

    mysql> SELECT ROUND(150.000,2), ROUND(150,2);
+------------------+--------------+
| ROUND(150.000,2) | ROUND(150,2) |
+------------------+--------------+
|           150.00 |          150 |
+------------------+--------------+

    Before MySQL 5.0.3, the behavior of ROUND() when the argument is halfway between two integers depends on the C library implementation. Different implementations round to the nearest even number, always up, always down, or always toward zero. If you need one kind of rounding, you should use a well-defined function such as TRUNCATE() or FLOOR() instead.

    As of MySQL 5.0.3, ROUND() uses the following rules depending on the type of the first argument:

    • For exact-value numbers, ROUND() uses the “round half up” or “round toward nearest” rule: A value with a fractional part of .5 or greater is rounded up to the next integer if positive or down to the next integer if negative. (In other words, it is rounded away from zero.) A value with a fractional part less than .5 is rounded down to the next integer if positive or up to the next integer if negative.

    • For approximate-value numbers, the result depends on the C library. On many systems, this means that ROUND() uses the "round to nearest even" rule: A value with any fractional part is rounded to the nearest even integer.

    The following example shows how rounding differs for exact and approximate values: 

    mysql> SELECT ROUND(2.5), ROUND(25E-1);
+------------+--------------+
| ROUND(2.5) | ROUND(25E-1) |
+------------+--------------+
| 3          |            2 |
+------------+--------------+

    For more information, see Section 11.13, “Precision Math”.

  • SIGN(X)

    Returns the sign of the argument as -1, 0, or 1, depending on whether X is negative, zero, or positive. 

    mysql> SELECT SIGN(-32);
        -> -1
mysql> SELECT SIGN(0);
        -> 0
mysql> SELECT SIGN(234);
        -> 1

  • SIN(X)

    Returns the sine of X, where X is given in radians. 

    mysql> SELECT SIN(PI());
        -> 1.2246063538224e-16
mysql> SELECT ROUND(SIN(PI()));
        -> 0

  • SQRT(X)

    Returns the square root of a nonnegative number X. 

    mysql> SELECT SQRT(4);
        -> 2
mysql> SELECT SQRT(20);
        -> 4.4721359549996
mysql> SELECT SQRT(-16);
        -> NULL

  • TAN(X)

    Returns the tangent of X, where X is given in radians. 

    mysql> SELECT TAN(PI());
        -> -1.2246063538224e-16
mysql> SELECT TAN(PI()+1);
        -> 1.5574077246549

  • TRUNCATE(X,D)

    Returns the number X, truncated to D decimal places. If D is 0, the result has no decimal point or fractional part. D can be negative to cause D digits left of the decimal point of the value X to become zero. 

    mysql> SELECT TRUNCATE(1.223,1);
        -> 1.2
mysql> SELECT TRUNCATE(1.999,1);
        -> 1.9
mysql> SELECT TRUNCATE(1.999,0);
        -> 1
mysql> SELECT TRUNCATE(-1.999,1);
        -> -1.9
mysql> SELECT TRUNCATE(122,-2);
       -> 100
mysql> SELECT TRUNCATE(10.28*100,0);
       -> 1028

    All numbers are rounded toward zero.


User Comments

Posted by [name withheld] on November 28 2002 12:27am[Delete] [Edit]

My brother had a case where he wanted to sort
randomly but ALSO use LIMIT so he could page
results - of course random will be different each time.

He wanted a random order that was not random for
the same session; so here is the idea:

In the web-side code calculate a numeric value which
is likely to stay the same for a session, perhaps
based on some session id, or timed-expiring cookie
value, etc, or from short-term stable HTTP headers.

Also require a numeric and well distributed value for
each record (doesn't have to be unique but works
well if it is).

Then:

... order by rand(numeric_field + session_value)
LIMIT blah;

So we see the ordering is preserved as
numeric_field+session_value will be the same for a
session, and numeric_field + session value are NOT
the same from row to row so we still get random
ordering.

Sam Liddicott

Posted by Hyungjin Ahn on January 9 2003 5:17pm[Delete] [Edit]

I might be caused by compiler ability to count to upto 30 places under zero. Win32 mysql probably mighe be compiled with 32bit compiler rather than 64bit. -- Hyungjin Ahn(ahj6@hotmail.com)

Posted by Girish Kshirsagar on November 27 2003 8:06pm[Delete] [Edit]

You may need to compare columns in databases after converting say a string column to a numeric column. These comparisons are automatic

Example
in the WHERE clause you may have to do something like this

oem.oem_id=substring(sku,5,3)

Here sku is a string who substring starting from location 5 from left and then having total length of 3 is compared with a numeric value of oem_id to satisfy the WHERE clause.

For more details see
http://www.bitmechanic.com/mail-archives/mysql/May1997/0494.html


Posted by [name withheld] on December 8 2003 1:01pm[Delete] [Edit]

I wanted to round to the nearest 0 or 5 cents in currency and this query worked:
select round((((cost*100) - (cost*100)%5) /100), 2) from SessionCost;

Posted by [name withheld] on December 22 2003 4:27am[Delete] [Edit]

If "SELECT * FROM tab ORDER BY RAND()" doesn't work for you. Try to put a random value between the brackets.


Posted by Justin Laing on January 6 2005 9:45am[Delete] [Edit]

Here is my work around for MySQL rounding issues (On most systems it rounds to the nearest even number on 5). This mess of a calculation will round up always in mysql, which is how most people in the united states think about rounding:

num = the number you are rounding

ROUND( TRUNCATE(num,2) + REPLACE( ( (num*1000) - ( TRUNCATE(num,2) *1000) / 1000, '5', '6'), 2)

This example rounds to 2 decimal places. If you want to round to three decimals just switch out the 2s for 3s and the 1000s for 10000s, etc.

It basically works by replacing all the fives beyond the two decimal places with sixes, which will always round up. Then calling the round function.

Posted by Joel Maxson on January 22 2005 5:38pm[Delete] [Edit]

Another way to round up to two decimals is using the following formula:
floor(num * 100 + .55)/100

Posted by Tim White on February 18 2005 7:48pm[Delete] [Edit]

This may be self-evident but:
In a list where some elements had priority and others not I needed to randomise the prioritised items and not the rest. The prioritised entries all had a value of 1 in a field called 'enhanced' and all entries had an abbreviated name ('abbrev') that they were otherwise sorted by. Using
ORDER BY (RAND() * enhanced) desc, abbrev
I could change the order of the enhanced listings yet maintain an alphabetical listing thereafter.

Posted by Alejandro Vargas on September 27 2005 11:53am[Delete] [Edit]

WARNING WITH ROUND AND FORMAT FUNCTIONS:

As mentioned in the manual, ROUND function has problems with values near to the limit values. The same prblem is found in the format function Let's see it:

round(1.15,1)=1.2 OK
round(1.25,1)=1.2 BAD, sould be 1.3
round(1.35,1)=1.4 OK
round(1.45,1)=1.4 BAD, sould be 1.5
round(1.55,1)=1.6 OK

And so on...

A walkarround for this sould be to use truncate adding 0,06. The same problem in found in the format function.

+---------------+----------------+-----------------------+
| round(1.45,1) | FORMAT(1.45,1) | truncate(1.45+0.06,1) |
+---------------+----------------+-----------------------+
|           1.4 | 1.4            |                   1.5 |
+---------------+----------------+-----------------------+

Of corse, if you want to use more than one digit, you should add as many 0 as you need to de value added in the truncate function. Note that in case of using 2 digits, the result of format is correct but round stills failing. It is more reliable to do the calculation using your own formula, with truncate.

+----------------+-----------------+-------------------------+
| round(1.145,2) | FORMAT(1.145,2) | truncate(1.145+0.006,2) |
+----------------+-----------------+-------------------------+
|           1.14 | 1.15            |                    1.15 |
+----------------+-----------------+-------------------------+

Posted by Veerappan Kannan on January 3 2006 9:16am[Delete] [Edit]

I think the "bads" are actually bankers rounding

Posted by Craig Martin on January 10 2006 2:31am[Delete] [Edit]

As truncate comment above, but negative number safe:

Take special care when using the the unsafe version with grouping functions like SUM(), as the end result can be way off if there is a big mix of negative/positive numbers.

sign(num) * truncate(abs(num)+0.06,1)

E.g...

+-----------------------+------------------------+-------------------------------------------+
| truncate(1.45+0.06,1) | truncate(-1.45+0.06,1) | sign(-1.45) * truncate(abs(-1.45)+0.06,1) |
+-----------------------+------------------------+-------------------------------------------+
|                   1.5 |        WRONG -->  -1.3 |                         CORRECT -->  -1.5 |
+-----------------------+------------------------+-------------------------------------------+

Posted by Ken Halsted on January 25 2006 4:28pm[Delete] [Edit]

I finally had to come up with my own solution for rounding with currency in the U.S.

Most of us consider this:

25.725 to be 25 dollars and 73 cents

But mysql was returning: round(25.725,2) as 25.72 which was throwing off my calc.

So, my workaround after not finding a solution is:

if num=25.725
============================
truncate(num + 0.0051,2)
============================

will yield this result: 25.73, which is correct.

I hope this helps someone else.

Ken

Posted by Justin Laing on March 2 2006 9:17pm[Delete] [Edit]

The rounding functions above are a little bit off from what most people would consider standard rounding.

If you use 6 as the number you are adding to the digit beyond significance then you will be rounding up 0.4s as well as 0.5s.

Here is my method:
rounding to two decimals
TRUNCATE(num + (SIGN(num) * 0.005), 2)
example 1
TRUNCATE(0.004 + (SIGN(0.004) * 0.005),2) = TRUNCATE(0.009,2) = 0.00
example 2
TRUNCATE(0.005 + (SIGN(0.005) * 0.005),2) = TRUNCATE(0.010,2) = 0.01

for three decimals it would be
TRUNCATE(num + (SIGN(num) * 0.0005), 3)
etc.

BTW this seems to be how PHP's round function works, so if you are trying to get calculations in PHP to match MySQL this is how I did it.

Posted by Tim Reynolds on April 15 2006 7:01pm[Delete] [Edit]

Am I mistaken about the command for an integer ranged RAND function...

Given what is printed here:
FLOOR(i + RAND() * (j - i))
I only ever get results in the range of i to j-1.
Shouldn't it be
FLOOR(i + RAND() * (j - i + 1 )) ?
I am getting results in the range I need with that. Maybe I am missing something, maybe once in a great while there will be a result that is j+1 and I have just not seen it.

BTW, I am using it as:
CREATE FUNCTION IRAND(param1 INT, param2 INT) RETURNS INT
RETURN FLOOR(param1 + RAND() * (param2-param1+1)) ;

Posted by Hans on May 31 2006 11:02am[Delete] [Edit]

That is true. The RAND() function returns a value 0.0 <= x <= 1.0
Thus, the values '0.0' and '1.0' can be returned althoug the changes are very very little.
In the example, where one wants a value between 7 and 12 inclusive, the value of '12' will hardly ever be returned.
I wanted a value of '0' or '1' (i.e. yes or no), so I used FLOOR(RAND() + 0.5), cuz if I'd used FLOOR(i + RAND() * (j – i), i.e. FLOOR(0 + RAND() * (1 – 0)) which evaluates to (FLOOR(RAND()), I would have gotten only one '1' and a trillillizillion 0's.



Posted by Florian Paulus on July 11 2006 9:40am[Delete] [Edit]

ROUND(X,Y)

ok i experienced like the description says different behaviour on rounding on different systems

so based on the examples by other ppl who might work for their issue but are neither save nor a
general purpose solution i have come up with my own solution for rounding up on 5

the number of decimal places you want : X
number : Y

general solution :

TRUNCATE((Y+SIGN(Y)*(POW(10,(1-X))/18)),X)


example (the other solutions fail here) :

y = 12.449
x = 1
result : 12.5

hope this helps you too

Posted by James Puddicombe on February 1 2007 2:44am[Delete] [Edit]

Simple but effective function for rounding to two decimals correctly (eg. 0.625 rounds to 0.63), unlike with the broken 'round' function

CREATE FUNCTION `v_round`(round_me DOUBLE)
RETURNS decimal(10,2)
DETERMINISTIC
SQL SECURITY DEFINER
COMMENT ''
BEGIN
return round_me;
END;

Posted by michael brenden on February 13 2007 9:50pm[Delete] [Edit]

Since using MySQL's RAND() function on a large rowset is notoriously slow:

To quickly select a random row, basically, do it in two SELECTS:

1. first SELECT finds out number of rows available, usnig a WHERE clause if desired.

2. web code chooses a random row from the number of rows (from step 1.) and saves this number in $x.

3. second SELECT (using the same WHERE clause in step 1.) uses LIMIT 1,$x.

Posted by József Rekedt-Nagy on March 28 2007 5:49am[Delete] [Edit]

Actually Order by Rand() Limit(1,X) won't work on larger sets, as it has to read through X-1 records to return the 1 you need.
In avarage it means reading through <NumberOfRecords>/2 records every time, thus it's slow.

Posted by Nate Wiger on April 8 2007 7:12pm[Delete] [Edit]

Yes, using limit is a silly way of doing that. Why not just select with id = the random id you picked?

Here's some Ruby:

max = dbh.query("select max(id) from table").fetch_row.first
rand_id = rand(max)
row = dbh.query("select * from table where id = #{rand_id}").fetch_hash
puts "Fetched: #{row['id']}"


Posted by Hero Ulster Magoncia on April 13 2007 8:11am[Delete] [Edit]

Nate:
Doing it that way doesn't work for everyone, some id values less than the max id might no longer exist in the table due to deletes.

Here's a simple solution (in php):

mysql_query('START TRANSACTION');

$count=mysql_fetch_row(mysql_query('SELECT COUNT(*) FROM table'));

$randomRow=mysql_fetch_row(mysql_query('SELECT * FROM table LIMIT '.(mt_rand(0,$count[0]-1)).',1'));

mysql_query('ROLLBACK');

Haven't really tested it, but you'd get the idea.
It's similar to michael's idea (posted above), only he had the limit parameters in the wrong order.

Posted by Mike McKee on February 11 2008 7:40am[Delete] [Edit]

Instead of using RAND and LIMIT tricks for randomness, with their limitations on speed, if you have a primary key that's an auto-incrementer, you could do it with these two SELECT like so:

SELECT MAX(pkey) FROM articles;

...then grab a random number (shown as $r below) between 1 and max in your code. Now return back to SQL like so:

SELECT * FROM articles where pkey > $r LIMIT $limit;

...where $limit is the number of rows you want to likely return.

...Then, to create the illusion of more randomness, just use an ORDER BY clause on the second SELECT above based on something arbitrary. For instance, if 'articles' has a column like author name and another like category, you could change the SELECT statement above like:

SELECT * FROM articles where pkey > $r ORDER BY category, name LIMIT $limit;

...So, by using this strategy, it's faster than having to randomly determine your pkeys and selecting only one record at a time.

In my case, I wanted to sort classified listings with some close approximation of randomness in order to rotate the listings, and this strategy has worked for me.

Posted by Gerhard Wolkerstorfer on April 4 2008 9:55am[Delete] [Edit]

After I had problems with the ROUND() function in an accounting application where i need commercial rounding I wrote this stored function that works very well for my needs:

CREATE FUNCTION ROUND_COMMERCIAL(value DOUBLE, preci INT(11)) RETURNS DOUBLE NO SQL
RETURN TRUNCATE((value * POW(10, preci)) + (IF(value = 0, 1,(value / ABS(value)))*(0.5 * POW(1, preci*-1))), 0) / POW(10, preci);

Posted by Szot Kamil on September 23 2008 12:03pm[Delete] [Edit]

If you want to select more records randomly you can use following method:

SET @toGet=10;
SET @left=(SELECT COUNT(*) FROM tableName)+1;

SELECT *, @toGet:=@toGet-1
FROM tableName
WHERE (@left:=@left-1)>0 AND RAND()<@toGet/@left;

It's much faster than ORDER BY RAND() LIMIT 10 (especially if you want to fetch small random subset of rows stored in table) but if it happens to return same set of rows, it returns them always in same order. If you want them to have random order then you have to scramble them after fetching using subquery:

SET @toGet=10;
SET @left=(SELECT COUNT(*) FROM tableName)+1;
SELECT * FROM (
SELECT *, @toGet:=@toGet-1
FROM tableName
WHERE (@left:=@left-1)>0 AND RAND()<@toGet/@left
) t ORDER BY RAND()

or on client side.

Posted by Dave Turner on February 2 2009 9:39pm[Delete] [Edit]

Oddly truncate produces 2 different results for the same mathematical function with variations in placement of truncate in relation to a conditional:

mysql> select version(),if(2.268>1.249,truncate(ceil(((2.268-1.249)/6+.01)*100)/100,2),'0.00') as fscpm;

+------------+-------+
| version()  | fscpm |
+------------+-------+
| 5.0.27-log | 0.18  | 
+------------+-------+

mysql> select version(),truncate(if(2.268>1.249,ceil(((2.268-1.249)/6+.01)*100)/100,'0.00'),2) as fscpm;
+------------+-------+
| version()  | fscpm |
+------------+-------+
| 5.0.27-log |  0.17 | 
+------------+-------+

Posted by Matthew Flaschen on March 28 2009 7:52am[Delete] [Edit]

Nearest even rounding is not bad. In fact, it results in less statistically biased results than always rounding up.

Posted by Guy Gordon on April 18 2009 6:38pm[Delete] [Edit]

If you reached this page looking for functions like MIN(a,b,...) and MAX(a,b,...) they are named LEAST()and GREATEST(), and are in section 11.2.3. Comparison Functions and Operators.

Posted by Mike N on March 9 2010 4:47pm[Delete] [Edit]

For those of you who need to implement banker's rounding in MySQL (handy if you're doing invoice reports and the numbers need to match up with accounting software like Simply Accounting that use banker's rounding), this is what I use:

CREATE FUNCTION BROUND( value DECIMAL(65,30), places TINYINT(3) UNSIGNED ) RETURNS DECIMAL(65,30) DETERMINISTIC RETURN
CASE WHEN
LOCATE( '.', value ) >= 1
AND LENGTH( value ) < 33
AND places > 0
AND LENGTH( value ) - LOCATE( '.', value ) > places
AND SUBSTRING( value, LOCATE( '.', value ) + places + 1, 1 ) = 5
AND SUBSTRING( value, LOCATE( '.', value ) + places, 1 ) % 2 = 0

THEN
SUBSTRING( value, 1, LOCATE( '.', value ) + places )
ELSE
ROUND( value, places )
END;

I've tested it with numbers like 0.051525354555657585950515253545 and it appears to operate properly to 30 decimal places, after this it falls back to the MySQL built-in ROUND() function.

Do your own testing first of course.

Posted by Mike N on March 10 2010 11:10pm[Delete] [Edit]

on my previous comment, it appears to short you one decimal place when doing negative numbers, you may want to (easily) correct this by ORing the length < 33 condition to be 34 for negative values.

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