MySQL, on most machines, uses 64-bit IEEE 754 floating point arithmetic for its calculations.
In integer contexts it uses integer arithmetic.
RAND()is not a perfect random number generator. It is mainly used to quickly generate pseudorandom numbers
MySQL provides the following arithmetic operators
If the numbers in your arithmetic are all integers, MySQL uses the
BIGINT (signed 64-bit) integer data type to do its work. For example:
select (1024 * 1024 * 1024 * 1024 *1024 * 1024) + 1 -> 1,152,921,504,606,846,977
select (1024 * 1024 * 1024 * 1024 *1024 * 1024 * 1024 ->
BIGINT out of range error
If any numbers in your arithmetic are fractional, MySQL uses 64-bit IEEE 754 floating point arithmetic. You must be careful when using floating point arithmetic, because many floating point numbers are, inherently, approximations rather than exact values.
The following returns the value of
PI formatted to 6 decimal places. The actual value is good to
SELECT PI(); -> 3.141593
Angles are in Radians, not Degrees. All computations are done in IEEE 754 64-bit floating point. All floating point computations are subject to small errors, known as machine ε (epsilon) errors, so avoid trying to compare them for equality. There is no way to avoid these errors when using floating point; they are built in to the technology.
If you use
DECIMAL values in trigonometric computations, they are implicitly converted to floating point, and then back to decimal.
Returns the sine of a number X expressed in radians
SELECT SIN(PI()); -> 1.2246063538224e-16
Returns the cosine of X when X is given in radians
SELECT COS(PI()); -> -1
Returns the tangent of a number X expressed in radians. Notice the result is very close to zero, but not exactly zero. This is an example of machine ε.
SELECT TAN(PI()); -> -1.2246063538224e-16
Returns the arc cosine of X if X is in the range
-1 to 1
SELECT ACOS(1); -> 0 SELECT ACOS(1.01); -> NULL
Returns the arc sine of X if X is in the range
-1 to 1
SELECT ASIN(0.2); -> 0.20135792079033
ATAN(x) returns the arc tangent of a single number.
SELECT ATAN(2); -> 1.1071487177941
ATAN2(X, Y) returns the arc tangent of the two variables X and Y. It is similar to calculating the arc tangent of Y / X. But it is numerically more robust: t functions correctly when X is near zero, and the signs of both arguments are used to determine the quadrant of the result.
Best practice suggests writing formulas to use
ATAN2() rather than
ATAN() wherever possible.
ATAN2(1,1); -> 0.7853981633974483 (45 degrees) ATAN2(1,-1); -> 2.356194490192345 (135 degrees) ATAN2(0, -1); -> PI (180 degrees) don't try ATAN(-1 / 0)... it won't work
Returns the cotangent of X
SELECT COT(12); -> -1.5726734063977
SELECT RADIANS(90) -> 1.5707963267948966 SELECT SIN(RADIANS(90)) -> 1 SELECT DEGREES(1), DEGREES(PI()) -> 57.29577951308232, 180
For exact numeric values (e.g.
DECIMAL): If the first decimal place of a number is 5 or higher, this function will round a number to the next integer away from zero. If that decimal place is 4 or lower, this function will round to the next integer value closest to zero.
SELECT ROUND(4.51) -> 5 SELECT ROUND(4.49) -> 4 SELECT ROUND(-4.51) -> -5
For approximate numeric values (e.g.
DOUBLE): The result of the
ROUND() function depends on the C library; on many systems, this means that
ROUND() uses the round to the nearest even rule:
SELECT ROUND(45e-1) -> 4 -- The nearest even value is 4 SELECT ROUND(55e-1) -> 6 -- The nearest even value is 6
To round up a number use either the
SELECT CEIL(1.23) -> 2 SELECT CEILING(4.83) -> 5
To round down a number, use the
SELECT FLOOR(1.99) -> 1
FLOOR and CEIL go toward / away from -infinity:
SELECT FLOOR(-1.01), CEIL(-1.01) -> -2 and -1 SELECT FLOOR(-1.99), CEIL(-1.99) -> -2 and -1
SELECT ROUND(1234.987, 2) -> 1234.99 SELECT ROUND(1234.987, -2) -> 1200
The discussion of up versus down and "5" applies, too.
To raise a number
x to a power
y, use either the
SELECT POW(2,2); => 4 SELECT POW(4,2); => 16
SQRT() function. If the number is negative,
NULL will be returned
SELECT SQRT(16); -> 4 SELECT SQRT(-3); -> NULL
To generate a pseudorandom floating point number between
1, use the
Suppose you have the following query
SELECT i, RAND() FROM t;
This will return something like this
To generate a random number in the range a <= n <= b, you can use the following formula
FLOOR(a + RAND() * (b - a + 1))
For example, this will generate a random number between 7 and 12
SELECT FLOOR(7 + (RAND() * 6));
A simple way to randomly return the rows in a table:
SELECT * FROM tbl ORDER BY RAND();
These are pseudorandom numbers.
The pseudorandom number generator in MySQL is not cryptographically secure. That is, if you use MySQL to generate random numbers to be used as secrets, a determined adversary who knows you used MySQL will be able to guess your secrets more easily than you might believe.
Return the absolute value of a number
SELECT ABS(2); -> 2 SELECT ABS(-46); -> 46
sign of a number compares it to 0.
|-1||n < 0|
|0||n = 0|
|1||n > 0|
SELECT SIGN(-423421); -> -1