Floor, Round, and Ceiling

I saw an interesting question today about rounding. Specifically they always wanted to round down. Now as it happens that wasn’t really what they needed, but we can get to that later. It did get me thinking and I figured it would make a good post.

ROUND

This is the most complicated of the three. It does a standard rounding. If value is .5 or over then you get back 1. If it’s less than .5 you get back 0. On top of that you get to pass the place you want to round to. So for example 0 rounds to the nearest ones place, -1 rounds to the tens place, 2 rounds to the hundredths.

SELECT 
	ROUND(13.567,0) 	-- 14.000  -- Ones place
	ROUND(13.567,-1),	-- 10.000  -- Tens place
	ROUND(13.567,2) 	-- 13.570  -- Hundredths place

 
Then on top of that you can pass a third optional parameter that causes it to truncate to that position rather than round.

SELECT 
	ROUND(13.567,0,1)	-- 13.000  -- Ones place
	ROUND(13.567,-1,1),	-- 10.000  -- Tens place
	ROUND(13.567,2,1) 	-- 13.560  -- Hundredths place

 
The value returned is a similar data type as the one passed in. Same precision and scale but TINYINT becomes INT for example.

Oddly ROUND(x,0,1) returns almost the same value as FLOOR.
 

FLOOR

Floor truncates to the nearest ones place just like ROUND(x,0,1). But while ROUND returns the same scale (where possible) as the data type passed in, the data type FLOOR returns has a 0 scale (where possible).

SELECT 
	FLOOR(13.5),	-- 13
	FLOOR(13.8),	-- 13
	FLOOR(13.2) 	-- 13

CEILING

Ceiling on the other hand is the opposite. It returns the same data type as floor (0 scale where possible) but returns the integer equal to or higher than the value passed in.

SELECT 
	CEILING(13.5),	-- 14
	CEILING(13.8),	-- 14
	CEILING(13.2) 	-- 14

Rounding to other values

Now back to that original question!

The OP (original poster) was actually wanting to round to the nearest 500,000. So how do we do that? 100,000 wouldn’t be hard at all but rounding to the nearest 5 is a bit more difficult. Well, ok, not that much more difficult. Simply divide by the value you want to round to, round, then multiply the value back.

Probably easier to see in action

SELECT
	ROUND(12345/500.0,0)*500, -- 12500
	ROUND(12345/300.0,0)*300, -- 12300
	ROUND(12345/700.0,0)*700  -- 12600

 
One last note, in order for the ROUND to work correctly the value you divide by has to have a decimal. Without it you basically get a FLOOR.