Postgresql subtract comma separated string in one column from another column

user_14057142 0
Viewed 59

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!

postgresql sql
3 Answers

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

user_6535390

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;
user_3303150

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.

user_97581940
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