Postgresql subtract comma separated string in one column from another column - sql

Question

The format is like:
col1
col2
V1,V2,V3,V4,V5,V6
V4,V1,V6
V1,V2,V3
V2,V3
I want to create another column called col3 which contains the subtraction of two columns.
What I have tried:
UPDATE myTable
SET col3=(replace(col1,col2,''))
It works well for rows like row2 since the order of replacing patterns matters.
I was wondering if there's a perfect way to achieve the same goal for rows like row1.
So the desired output would be:
col1
col2
col3
V1,V2,V3,V4,V5,V6
V4,V1,V6
V2,V3,V5
V1,V2,V3
V2,V3
V1
Any suggestions would be appreciated!

Answer 1

Split values into tables, subtract sets and then assemble it back. Everything is possible as an expression defining new query column.
with t (col1,col2) as (values
('V1,V2,V3,V4,V5,V6','V4,V1,V6'),
('V1,V2,V3','V2,V3')
)
select col1,col2
, (
select string_agg(v,',')
from (
select v from unnest(string_to_array(t.col1,',')) as a1(v)
except
select v from unnest(string_to_array(t.col2,',')) as a2(v)
) x
)
from t
DB fiddle

Answer 2

You will have to unnest the elements then apply an EXCEPT clause on the "unnested" rows and aggregate back:
select col1,
col2,
(select string_agg(item,',' order by item)
from (
select *
from string_to_table(col1, ',') as c1(item)
except
select *
from string_to_table(col2, ',') as c2(item)
) t)
from the_table;
I wouldn't store that result in a separate column, but if you really need to introduce even more problems by storing another comma separated list.
update the_table
set col3 = (select string_agg(item,',' order by item)
from (
select *
from string_to_table(col1, ',') as c1(item)
except
select *
from string_to_table(col2, ',') as c2(item)
) t)
;
string_to_table() requires Postgres 14 or newer. If you are using an older version, you need to use unnest(string_to_array(col1, ',')) instead
If you need that a lot, consider creating a function:
create function remove_items(p_one text, p_other text)
returns text
as
$$
select string_agg(item,',' order by item)
from (
select *
from string_to_table(col1, ',') as c1(item)
except
select *
from string_to_table(col2, ',') as c2(item)
) t;
$$
language sql
immutable;
Then the above can be simplified to:
select col1, col2, remove_items(col1, col2)
from the_table;

Answer 3

Note, POSTGRESQL is not my forte, but thought I'd have a go at it. Try:
SELECT col1, col2, RTRIM(REGEXP_REPLACE(Col1,CONCAT('\m(?:', REPLACE(Col2,',','|'),')\M,?'),'','g'), ',') as col3 FROM myTable
See an online fidle.
The idea is to use a regular expession to replace all values, based on following pattern:
\m - Word-boundary at start of word;
(?:V4|V1|V6) - A non-capture group that holds the alternatives from col2;
\M - Word-boundary at end of word;
,? - Optional comma.
When replaced with nothing we need to clean up a possible trailing comma with RTRIM(). See an online demo where I had to replace the word-boundaries with the \b word-boundary to showcase the outcome.