How to Unnest Comma-Separated Values in PostgreSQL and Preserve Element Order?

Expansion with element numbers in PostgreSQL

Question:

Using the unnest() function to expand a column containing comma-separated values ​​will only return the elements themselves, not their original positions in the string.

Target:

Get the element and its sequence number in the source string.

Solution:

PostgreSQL 14 and above

Use string_to_table() function and WITH ORDINALITY clause:

<code class="language-sql">SELECT t.id, a.elem, a.nr
FROM tbl t
LEFT JOIN LATERAL string_to_table(t.elements, ',') WITH ORDINALITY AS a(elem, nr) ON true;</code>

PostgreSQL 9.4 and higher

Add WITH ORDINALITY clause to unnest() function:

<code class="language-sql">SELECT t.id, a.elem, a.nr
FROM tbl AS t
LEFT JOIN LATERAL unnest(string_to_array(t.elements, ',')) WITH ORDINALITY AS a(elem, nr) ON true;</code>

PostgreSQL 8.4 - 9.3

Omit the row_number() clause in the ORDER BY function:

<code class="language-sql">SELECT *, row_number() OVER (PARTITION by id) AS nr
FROM (SELECT id, regexp_split_to_table(elements, ',') AS elem FROM tbl) t;</code>

PostgreSQL 8.1 - 8.4

Create a custom function f_unnest_ord() to extract elements and their serial numbers:

<code class="language-sql">CREATE FUNCTION f_unnest_ord(anyarray, OUT val anyelement, OUT ordinality integer)
RETURNS SETOF record
LANGUAGE sql IMMUTABLE AS
'SELECT [i], i - array_lower(,1) + 1
FROM generate_series(array_lower(,1), array_upper(,1)) i';</code>

Then use this to expand values ​​with ordinal numbers:

<code class="language-sql">SELECT id, arr, (rec).*
FROM (SELECT *, f_unnest_ord(arr) AS rec FROM (VALUES (1, '{a,b,c}'::text[])...) t) sub;</code>

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