is it possible to ignore the error and continue execute update in PostgreSQL

All we need is an easy explanation of the problem, so here it is.

I have a table stored all my project rss channel url, now I found some url end with ‘/’ but some sub url are not. I my app I have to handle this situation in everywhere. Then I want to store all the sub url link without the last ‘/’, if the url end with ‘/’, I want to delete the end of ‘/’. I have write the update sql command like this:

UPDATE rss_sub_source 
SET sub_url = SUBSTRING(sub_url, 1, CHAR_LENGTH(sub_url) - 1) 
WHERE sub_url LIKE '%/';

when I execute the sql:

SQL Error [23505]: ERROR: duplicate key value violates unique constraint "unique_sub_url"
  Detail: Key (sub_url)=(https://physicsworld.com/feed) already exists.

the error shows that some url without ‘/’ have already exists. when I update wht end with ‘/’ url, it will conflict with the exists one because I add an uniq constraint. There table contains thousands of url, update one by one obviously impossible. So I want to ignore and jump to update the url if it did not obey the uniq constraint, only update the success record. Finnaly delete the end with ‘/’ record.

Is it possible to ignore the update error events in PostgreSQL? if not what should I do to make all rss url did not end with ‘/’?

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

You can check if such a sub_url already exists:

UPDATE rss_sub_source 
SET sub_url = SUBSTRING(sub_url, 1, CHAR_LENGTH(sub_url) - 1) 
WHERE sub_url LIKE '%/' 
AND NOT EXISTS 
(
  SELECT 1 FROM rss_sub_source 
  WHERE sub_url = SUBSTRING
                  (
                    sub_url, 1, CHAR_LENGTH(sub_url) - 1
                  ) 
)

Method 2

Based on your comment, you have to find the duplicate then delete those records first and then do the update.

create table rss_sub_source (
  id int,
  sub_url varchar(100)
  );
  
  insert into rss_sub_source 
  select 1,'https://www.google.com/'
  union all
  select 2,'https://www.msn.com/'
  union all
  select 3,'https://www.google.com/'
  union all 
  select 4,'https://www.gmail.com'

--find dupicates and delete

delete  FROM rss_sub_source r
using (
select SUBSTRING(sub_url, 1, CHAR_LENGTH(sub_url) - 1),
       row_number() OVER(partition by SUBSTRING(sub_url, 1,CHAR_LENGTH(sub_url) - 1) 
       order by id asc) rnk,
       id
from rss_sub_source
  )  x where x.id=r.id
 and rnk>1;
 
 select * FROM rss_sub_source;
 
 --final update 
 UPDATE rss_sub_source 
SET sub_url = SUBSTRING(sub_url, 1, CHAR_LENGTH(sub_url) - 1) 
WHERE sub_url LIKE '%/';

select * FROM rss_sub_source;

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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