PostgreSQL update query - sql

Question

I need to update table in my database. For sake of simplicity lets assume that table's name is tab and it has 2 columns: id (PRIMARY KEY, NOT NULL) and col (UNIQUE VARCHAR(300)). I need to update table this way:
id col
----------------------------------------------------
1 'One two three'
2 'One twothree'
3 'One two three'
4 'Remove white spaces'
5 'Something'
6 'Remove whitespaces '
to:
id col
----------------------------------------------------
1 'Onetwothree'
2 'Removewhitespaces'
3 'Something'
Id numbers and order of the rows after update is not important and can be different. I use PostgreSQL. Some of the columns are FOREIGN KEYs. That's why dropping UNIQUE constraint from col would be troublesome.

Answer 1

I think just using replace in this format will do what you want.
update tab
set col = replace(col, ' ', '');
Here's a SQLFiddle for it.

Answer 2

You shouldn't be using the non-descriptive column name id, even if some half-wit ORMs are in the habit of doing that. I use tab_id instead for this demo.
I interpret your description this way: You have other tables with FK columns pointing to tab.col. Like table child1 in my example below.
To clean up the mess, do all of this in a single session to preserve the temporary table I use. Better yet, do it all in a single transaction.
Update all referencing tables to have all referencing rows point to the "first" (unambiguously! - how ever you define that) in a set of going-to-be duplicates in tab.
Create a translation table up to be used for all updates:
CREATE TEMP TABLE up AS
WITH t AS (
SELECT tab_id, col, replace(col, ' ', '') AS col1
,row_number() OVER (PARTITION BY replace(col, ' ', '')
ORDER BY tab_id) AS rn
FROM tab
)
SELECT b.col AS old_col, a.col AS new_col
FROM (SELECT * FROM t WHERE rn = 1) a
JOIN (SELECT * FROM t WHERE rn > 1) b USING (col1);
Then update all your referencing tables.
UPDATE child1 c
SET col = up.new_col
FROM up
WHERE c.col = up.old_col;
-- more tables?
-> SQLfiddle
Now, all references point to the "first" in a group of dupes, and you have got your license to kill the rest.
Remove duplicate rows except the first from tab.
DELETE FROM tab t
USING up
WHERE t.col = up.old_col
Be sure that all referencing FK constraints have the ON UPDATE CASCADE clause.
ALTER TABLE child1 DROP CONSTRAINT child1_col_fkey;
ALTER TABLE child1 ADD CONSTRAINT child1_col_fkey FOREIGN KEY (col)
REFERENCES tab (col)
ON UPDATE CASCADE;
-- more tables?
Sanitize your values by removing white space
UPDATE tab
SET col = replace(col, ' ', '');
This only takes care of good old space characters (ASCII value 32, Unicode U+0020). Do you have others?
All FK constraints should be pointing to tab.tab_id to begin with. Your tables would be smaller and faster and all of this would be easier.

Answer 3

I solved it much easier then Erwin. I don't have SQL on my computer to test it but something like that worked for me:
DELETE FROM tab WHERE id IN (
SELECT id FROM (
SELECT id, col, row_number() OVER (PARTITION BY regexp_replace(col, '[ \t\n]*', '')) AS c WHERE c > 1;
)
)
UPDATE tab SET col = regexp_replace(col, '[ \t\n]*', '');