Get columns that differ between 2 rows - postgresql

Question

I have a table company with 60 columns. The goal is to create a tool to find, compare and eliminate duplicates in this table.
Example: I find 2 companies that potentially are the same, but I need to know which values (columns) differ between these 2 rows in order to continue.
I think it is possible to compare column by column x 60, but I search for a simpler and more generic solution.
Something like:
SELECT * FROM company where co_id=22
SHOW DIFFERENCE
SELECT * FROM company where co_id=33
The result should be the column names that differ.

Answer 1

For this you may use an intermediate key/value representation of the rows, with JSON functions or alternatively with the hstore extension (now only of historical interest). JSON comes built-in with every reasonably recent version of PostgreSQL, whereas hstore must be installed in the database with CREATE EXTENSION.
Demo:
CREATE TABLE table1 (id int primary key, t1 text, t2 text, t3 text);
Let's insert two rows that differ by the primary key and one other column (t3).
INSERT INTO table1 VALUES
(1,'foo','bar','baz'),
(2,'foo','bar','biz');
Solution with json
First with get a key/value representation of the rows with the original row number, then we pair the rows based on their original row number and
filter out those with the same "value" column
WITH rowcols AS (
select rn, key, value
from (select row_number() over () as rn,
row_to_json(table1.*) as r from table1) AS s
cross join lateral json_each_text(s.r)
)
select r1.key from rowcols r1 join rowcols r2
on (r1.rn=r2.rn-1 and r1.key = r2.key)
where r1.value <> r2.value;
Sample result:
key
-----
id
t3
Solution with hstore
SELECT skeys(h1-h2) from
(select hstore(t.*) as h1 from table1 t where id=1) h1
CROSS JOIN
(select hstore(t.*) as h2 from table1 t where id=2) h2;
h1-h2 computes the difference key by key and skeys() outputs the result as a set.
Result:
skeys
-------
id
t3
The select-list might be refined with skeys((h1-h2)-'id'::text) to always remove id which, as the primary key, will obviously always differ between rows.

Answer 2

Here's a stored procedure that should get you most of the way...
While this should work "as is", it has no error checking, which you should add.
It gets all the columns in the table, and loops over them. A difference is when the count of the distinct items is more than one.
Also, the output is:
The count of the number of differences
Messages for each column where there is a difference
It might be more useful to return a rowset of the columns with the differences. Anyway, good luck!
Usage:
SELECT showdifference('public','company','co_id',22,33)
CREATE OR REPLACE FUNCTION showdifference(p_schema text, p_tablename text,p_idcolumn text,p_firstid integer, p_secondid integer)
RETURNS INTEGER AS
$BODY$
DECLARE
l_diffcount INTEGER;
l_column text;
l_dupcount integer;
column_cursor CURSOR FOR select column_name from information_schema.columns where table_name = p_tablename and table_schema = p_schema and column_name <> p_idcolumn;
BEGIN
-- need error checking here, to ensure the table and schema exist and the columns exist
-- Should also check that the records ids exist.
-- Should also check that the column type of the id field is integer
-- Set the number of differences to zero.
l_diffcount := 0;
-- use a cursor to iterate over the columns found in information_schema.columns
-- open the cursor
OPEN column_cursor;
LOOP
FETCH column_cursor INTO l_column;
EXIT WHEN NOT FOUND;
-- build a query to see if there is a difference between the columns. If there is raise a notice
EXECUTE 'select count(distinct ' || quote_ident(l_column) || ' ) from ' || quote_ident(p_schema) || '.' || quote_ident(p_tablename) || ' where ' || quote_ident(p_idcolumn) || ' in ('|| p_firstid || ',' || p_secondid ||')'
INTO l_dupcount;
IF l_dupcount > 1 THEN
-- increment the counter
l_diffcount := l_diffcount +1;
RAISE NOTICE '% has % differences', l_column, l_dupcount ; -- for "real" you might want to return a rowset and could do something here
END IF;
END LOOP;
-- close the cursor
CLOSE column_cursor;
RETURN l_diffcount;
END;
$BODY$
LANGUAGE plpgsql VOLATILE STRICT
COST 100;