Round to n Significant Figures in SQL - 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
I am aware of the ROUND() function, which rounds to n decimal places rather than significant figures.

Answer 1

select round(#number,#sf-1- floor(log10(abs(#number)))) should do the trick !
Successfully tested on your two examples.
Edit : Calling this function on #number=0 won't work. You should add a test for this before using this code.
create function sfround(#number float, #sf int) returns float as
begin
declare #r float
select #r = case when #number = 0 then 0 else round(#number ,#sf -1-floor(log10(abs(#number )))) end
return (#r)
end

Answer 2

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)

Answer 3

I think I've managed it.
CREATE FUNCTION RoundSigFig(#Number float, #Figures int)
RETURNS float
AS
BEGIN
DECLARE #Answer float;
SET #Answer = (
SELECT
CASE WHEN intPower IS NULL THEN 0
ELSE FLOOR(fltNumber * POWER(CAST(10 AS float), intPower) + 0.5)
* POWER(CAST(10 AS float), -intPower)
END AS ans
FROM (
SELECT
#Number AS fltNumber,
CASE WHEN #Number > 0
THEN -((CEILING(LOG10(#Number)) - #Figures))
WHEN #Number < 0
THEN -((FLOOR(LOG10(#Number)) - #Figures))
ELSE NULL END AS intPower
) t
);
RETURN #Answer;
END

Answer 4

You could divide by 100 before rounding and then multiplying by 100...