Replace values in array column with related values from another table - sql

Question

In my database I have a table relations with a column relation_ids containing the IDs of users (user_id). This takes the form of an array with many IDs possible, e.g.:
{111,112,156,4465}
I have another table names containing information on users such as user_id, first_name, last_name etc.
I would like to create an SQL query to return all rows from relations with all columns, but append the array column relation_ids with first_name from the names table substituted for IDs.
Is it possible as some kind of subquery?

Answer 1

Assuming you want to preserve the order in the array - first names listed in the same order as IDs in the original relation_ids.
I suggest an ARRAY constructor over a correlated subquery with unnest() and WITH ORDINALITY, joined to the names table, like:
SELECT r.*
, (ARRAY (
SELECT n.first_name
FROM unnest(r.relation_ids) WITH ORDINALITY AS a(user_id, ord)
JOIN names n ON n.user_id = a.user_id
ORDER BY a.ord
)
) AS first_names
FROM relations r;
This query preserves all rows from relations in any case.
Corner cases to note:
1. A NULL value in relation_ids (for the whole column) is translated to an empty array. (Same as empty array in the source.)
2. NULL elements are silently dropped from the array.
You might want to define desired behavior if those corner cases are possible ...
db<>fiddle here
Related:
LEFT OUTER JOIN on array column with multiple values
PostgreSQL unnest() with element number
Considered a normalized db design:
Can PostgreSQL array be optimized for join?

Answer 2

This will get you all the columns and rows from Relations with first_name appended from the Names table.
Select Relations.relation_ids, Names.user_id, Names.first_name From Relations
Inner Join Names On Relations.user_id=Names.user_id