How to remove values from an array that exist in another array?

Question

In PostgreSQL I have a table (Table) that contains an id column (ID) and another column (Values) that contains an array of strings. I have a select query (SelectQuery) that gets me an ID that matches Table.ID, as well as an array of values (RemoveValues). I would like to now remove from the Values array, any strings that are contained in the RemoveValues array that match on ID, updating Table.

--Table--

ID            Values
--  ----------------------------
1   {string1, string2, string3}
2   {string1, string2, string3}
...

--SelectQuery--

ID          RemoveValues
--  ----------------------------
1   {string2}
2   {string1, string2}
...

Execute query that I can't figure out

--Table--

ID            Values
--  ----------------------------
1   {string1, string3}
2   {string3}
...

I can't seem to figure out a good way to do this, I've played with a number of joins and the sql array methods and haven't found anything that works. Is there's a way to loop the remove() method for each element in RemoveValues? I've also tried something like this with no luck:

select array_agg(elem)
from Table, unnest(Table.Values) elem
where elem <> all(
    SelectQuery )
);

I'm feeling like my remaining option is a bash script, unless anyone can suggest a path forward using SQL?

Erwin Brandstetter · Accepted Answer · 2022-02-21 01:15:24Z

There are many ways.

First, you have to define whether there can be duplicates or NULL values in either of the arrays - and how to deal with those if possible. And whether to preserve original order of elements.

Assuming either is possible, and these are the rules:

Count duplicates separately. So if Values has five elements 'foo' and RemoveValues has three elements 'foo', thee will be two element 'foo' in the result.
Treat NULL values equal. So NULL can be removed with NULL (though in other contexts NULL = NULL yields NULL).
Order of array elements does not have to be preserved.
<SelectQuery> produces unique ID values.

That matches the behavior of standard SQL EXCEPT ALL. See:

Using EXCEPT clause in PostgreSQL

So:

SELECT t.*, sub.result_array
FROM   tbl t
JOIN   (<SelectQuery>) s USING ("ID")
CROSS  JOIN LATERAL (
   SELECT ARRAY (
      SELECT unnest(t."Values")
      EXCEPT ALL
      SELECT unnest(s."RemoveValues")
      )
   ) sub(result_array);

About the LATERAL subquery:

What is the difference between LATERAL JOIN and a subquery in PostgreSQL?

Can be plain CROSS JOIN because an ARRAY constructor always produces a row.

You could wrap the functionality in a custom function:

CREATE FUNCTION f_arr_except_arr(a1 text[], a2 text[])
  RETURNS text[]
  LANGUAGE SQL IMMUTABLE PARALLEL SAFE
BEGIN ATOMIC
SELECT ARRAY (SELECT unnest(a1) EXCEPT ALL SELECT unnest(a2));
END;

Requires Postgres 14. See:

What does BEGIN ATOMIC ... END mean in a Postgresql SQL function / procedure?

For older (or any) Postgres versions, and for any array type:

CREATE OR REPLACE FUNCTION f_arr_except_arr(a1 anyarray, a2 anyarray)
  RETURNS anyarray
  LANGUAGE SQL IMMUTABLE PARALLEL SAFE AS
$func$
SELECT ARRAY (SELECT unnest(a1) EXCEPT ALL SELECT unnest(a2));
$func$;

db<>fiddle here

(PARALLEL SAFE only for Postgres 9.6 or later, though.)

Then your query can be something like:

SELECT *, f_arr_except_arr(t."Values", s."RemoveValues") AS result_values
FROM   tbl t
JOIN  (<SelectQuery>) s USING ("ID");

It's unclear whether you actually locked in CaMeL-case spelling with double-quotes. I assumed as much, but better you don't. See:

Are PostgreSQL column names case-sensitive?

And your UPDATE can be:

UPDATE tbl t
SET    "Values" = f_arr_except_arr(t."Values", s."RemoveValues")
FROM   <SelectQuery>) s
WHERE  s."ID" = t."ID"
AND    "Values" IS DISTINCT FROM f_arr_except_arr(t."Values", s."RemoveValues");

The additional WHERE clause is optional to avoid empty updates. See:

Erwin, as always you're a SQL legend. This answer was perfect and got me exactly what I needed. Un-nesting the array to remove values was brilliant, thank you!

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