Round to n Significant Figures in SQL

Question

I would like to be able to round a number to n significant figures in SQL. So:

123.456 rounded to 2sf would give 120
0.00123 rounded to 2sf would give 0.0012
         


        

Answer 1

Adapted the most popular answer by Brann to MySQL for those who come looking like me.

CREATE FUNCTION `sfround`(num FLOAT, sf INT) # creates the function
RETURNS float # defines output type
DETERMINISTIC # given input, will return same output

BEGIN

    DECLARE r FLOAT;  # make a variable called r, defined as a float

    IF( num IS NULL OR num = 0) THEN # ensure the number exists, and isn't 0
        SET r = num; # if it is; leave alone

    ELSE
        SET r = ROUND(num, sf - 1 - FLOOR(LOG10(ABS(num))));
    /* see below*/
    END IF;

    RETURN (r);

END

/* Felt too long to put in comment */

ROUND(num, sf - 1 - FLOOR(LOG10(ABS(num))))

• The part that does the work - uses ROUND function on the number as normal, but the length to be rounded to is calculated
• ABS ensures positive
• LOG10 gets the number of digits greater than 0 in the number
• FLOOR gets the largest integer smaller than the resultant number
• So always rounds down and gives an integer
• sf - 1 - FLOOR(...) gives a negative number

• works because ROUND(num, -ve num) rounds to the left of the decimal point

• For just a one off, ROUND(123.456, -1) and ROUND(0.00123,4) return the requested answers ((120, 0.0012)