Question

Given a string column that represents a path ('/11/22/33/44'), how do I return the next number after the given one.

For example:

• given an id=22, I want to return 33 from the path /11/22/33/44/.
• given an id=44, I want to return NULL from the path /11/22/33/44/.

This is the bit I use to convert the string into a set of records:

SELECT unnest(string_to_array(trim(both '/' from '/11/22/33/44/'), '/')::integer[]);
 unnest 
--------
     11
     22
     33
     44
(4 rows)

But how to obtain the “next entry” here (so given 22 -> 33)?

Answer 1

One way is a window function:

SELECT a FROM (
    SELECT *, lag(a) OVER () AS lag_1
    FROM unnest(string_to_array(trim(both '/' from '/11/22/33/44/'), '/')::integer[]) a
) b WHERE lag_1 = 22;

This solution returns zero rows for the 44 input, but that’s easily addressed by invoking it as a subquery.

Alternately, for integer arrays you can use the intarray extension‘s idx function:

CREATE EXTENSION intarray;

WITH x(arr) AS (
    SELECT string_to_array(trim(both '/' from '/11/22/33/44/'), '/') :: integer[]
)
SELECT arr[idx(arr,22)+1] FROM x;

to look up the next index in the array. intarray is an extension distributed with PostgreSQL, it’s not a 3rd party add-on. This solution produces a NULL result for 44 without further manipulation.