PostgreSQL ORDER BY issue - natural sort

One approach you can take is to create a naturalsort function for this. Here's an example, written by Postgres legend RhodiumToad.

create or replace function naturalsort(text)
    returns bytea language sql immutable strict as $f$
    select string_agg(convert_to(coalesce(r[2], length(length(r[1])::text) || length(r[1])::text || r[1]), 'SQL_ASCII'),'\x00')
    from regexp_matches($1, '0*([0-9]+)|([^0-9]+)', 'g') r;
$f$;

Source: http://www.rhodiumtoad.org.uk/junk/naturalsort.sql

To use it simply call the function in your order by:

SELECT * FROM employees ORDER BY naturalsort(em_code) DESC

The reason is that the string sorts alphabetically (instead of numerically like you would want it) and 1 sorts before 9. You could solve it like this:

SELECT * FROM employees
ORDER  BY substring(em_code, 3)::int DESC;

It would be more efficient to drop the redundant 'EM' from your em_code - if you can - and save an integer number to begin with.

Answer to question in comment

To strip any and all non-digits from a string:

SELECT regexp_replace(em_code, E'\\D','','g')
FROM   employees;

\D is the regular expression class-shorthand for "non-digits".
'g' as 4th parameter is the "globally" switch to apply the replacement to every occurrence in the string, not just the first.

After replacing every non-digit with the empty string, only digits remain.

Since Postgres 9.6, it is possible to specify a collation which will sort columns with numbers naturally.

https://www.postgresql.org/docs/10/collation.html

-- First create a collation with numeric sorting
CREATE COLLATION numeric (provider = icu, locale = 'en@colNumeric=yes');

-- Alter table to use the collation
ALTER TABLE "employees" ALTER COLUMN "em_code" type TEXT COLLATE numeric;

Now just query as you would otherwise.

SELECT * FROM employees ORDER BY em_code

On my data, I get results in this order (note that it also sorts foreign numerals):

This always comes up in questions and in my own development and I finally tired of tricky ways of doing this. I finally broke down and implemented it as a PostgreSQL extension:

https://github.com/Bjond/pg_natural_sort_order

It's free to use, MIT license.

Basically it just normalizes the numerics (zero pre-pending numerics) within strings such that you can create an index column for full-speed sorting au naturel. The readme explains.

The advantage is you can have a trigger do the work and not your application code. It will be calculated at machine-speed on the PostgreSQL server and migrations adding columns become simple and fast.