Re: Nice to have features: Percentage function

On 04/15/2017 10:47 PM, Ron Ben wrote:
> Hi,
> I'm always finiding myself writing many varations of functions to
> calculate percentage.
> I think it would be nice if postgresql would have build in functions for
> that.
> I think the major functionality is something like the 3 ooptions here:
> https://percentagecalculator.net/
>
> It may help to keep code simple and clean and it seem like something
> simple to implement.

Plan B, CREATE your own extension. What follows is my first attempt at
creating an extension and the functions included are simple placeholders
more then anything else:

File name: calc_percents--1.0.sql

-- complain if script is sourced in psql, rather than via CREATE EXTENSION
\echo Use "CREATE EXTENSION calc_percents" to load this file. \quit

CREATE OR REPLACE FUNCTION public.percent_of(val1 numeric, val2 numeric)
RETURNS numeric
LANGUAGE sql
AS $function$
select (val1 / 100) * val2;
$function$
;

CREATE OR REPLACE FUNCTION public.what_percent(val1 numeric, val2 numeric)
RETURNS numeric
LANGUAGE sql
AS $function$
SELECT (val1 / val2) * 100;
$function$
;

CREATE OR REPLACE FUNCTION public.percent_diff(val1 numeric, val2 numeric)
RETURNS numeric
LANGUAGE sql
AS $function$
select (val2 - val1) / val1 * 100;
$function$
;

File name: calc_percents.control

# calc_percents extension
comment = 'Functions for calculating percentages'
default_version = '1.0'
relocatable = true

Install the above in $SHARE/extension, in my case
/usr/local/pgsql/share/extension/

Then:

test=# create extension calc_percents;
CREATE EXTENSION
test=# \df percent_of
List of functions
Schema | Name | Result data type | Argument data types |
Type
--------+------------+------------------+----------------------------+--------
public | percent_of | numeric | val1 numeric, val2 numeric |
normal

test=# select * from round(percent_of(10, 100), 2) ;
round
-------
10.00

test=# \df percent_diff
List of functions
Schema | Name | Result data type | Argument data types
| Type
--------+--------------+------------------+----------------------------+--------
public | percent_diff | numeric | val1 numeric, val2 numeric
| normal

test=# select * from round(percent_diff(100, 109), 2) ;
round
-------
9.00
(1 row)

test=# \df what_percent
List of functions
Schema | Name | Result data type | Argument data types
| Type
--------+--------------+------------------+----------------------------+--------
public | what_percent | numeric | val1 numeric, val2 numeric
| normal
(1 row)

test=# select * from round(what_percent(10, 109), 2) ;
round
-------
9.17

>
> If you think it's a good idea it would be nice if someone can implement
> this.

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com