how to select non repeated date intervals?

My products table,

year
2014
2013
2012
2011
2010
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000

My Query,

SELECT dt||'-'||dt1 
FROM
(SELECT year as dt, year+4 as dt1
FROM
products 
GROUP BY year);

My output,

2000-2004
2001-2005
2002-2006
2003-2007
2004-2008
2005-2009
2006-2010
2007-2011
2008-2012
2009-2013
2010-2014
2011-2015
2012-2016
2013-2017
2014-2018

expected result,

2000-2004 
2005-2009 
2010-2014

I’m not getting any clue for generating that date interval. Any tips and suggestion of query will be of great help. Thanks

Answer

You can subtract a modulus 5 from the year and group by that.

SELECT (year-(year%5))||'-'||(year-(year%5)+4) AS years
FROM products 
GROUP BY year-(year%5)

A modulus N is the remainder when the nearest lower multitude of N is substracted from a number.
For example:

11%5 = 11 - 10 = 1 
8%5 = 8 - 5 = 3

So by subtracting it from the year you get the nearest multitude of 5

2018 - 2018%5 = 2018 - 3 = 2015

Another way to round the year down is to divide & multiply it by 5

SELECT (CAST(year/5 AS INTEGER)*5)||'-'||MAX(year) AS years
FROM products 
GROUP BY CAST(year/5 AS INTEGER)*5;

A test on db<>fiddle here