PostgreSQL ORDER BY issue - natural sort

I've got a Postgres ORDER BY issue with the following table:

em_code  name
EM001    AAA
EM999    BBB
EM1000   CCC

To insert a new record to the table,

1. I select the last record with SELECT * FROM employees ORDER BY em_code DESC
2. Strip alphabets from em_code usiging reg exp and store in ec_alpha
3. Cast the remating part to integer ec_num
4. Increment by one ec_num++
5. Pad with sufficient zeors and prefix ec_alpha again

When em_code reaches EM1000, the above algorithm fails.

First step will return EM999 instead EM1000 and it will again generate EM1000 as new em_code, breaking the unique key constraint.

Any idea how to select EM1000?

Solution 1:

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

Solution 2:

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.