Find duplicate values in oracle - sql

Question

I'm using this query to find duplicate values in a table:
select col1,
count(col1)
from table1
group by col1
having count (col1) > 1
order by 2 desc;
But also I want to add another column from the same table, like this:
select col1,
col2,
count(col1)
from table1
group by col1
having count (col1) > 1
order by 2 desc;
I get an ORA-00979 error with that second query
How can I add another column in my search?

Answer 1

Your query should be
SELECT * FROM (
select col1,
col2,
count(col1) over (partition by col1) col1_cnt
from table1
)
WHERE col1_cnt > 1
order by 2 desc;

Answer 2

Presumably you want to get col2 for each duplicate of col1 that turns up. You can't really do that in a single query^. Instead, what you need to do is get your list of duplicates, then use that to retrieve any other associated values:
select col1, col2
from table1
where col1 in (select col1
from table1
group by col1
having count (col1) > 1)
order by col2 desc
^ Okay, you can, by using analytic functions, as #rs. demonstrated. For this scenario, I suspect that the nested query will be more efficient, but both should give you the same results.
Based on comments, it seems like you're not clear on why you can't just add the second column. Assume you have sample data that looks like this:
Col1 | Col2
-----+-----
1 | A
1 | B
2 | C
2 | D
3 | E
If you run
select Col1, count(*) as cnt
from table1
group by Col1
having count(*) > 1
then your results will be:
Col1 | Cnt
-----+-----
1 | 2
2 | 2
You can't just add Col2 to this query without adding it to the group by clause because the database will have no way of knowing which value you actually want (i.e. for Col1=1 should the DB return 'A' or 'B'?). If you add Col2 to the group by clause, you get the following:
select Col1, Col2, count(*) as cnt
from table1
group by Col1, Col2
having count(*) > 1
Col1 | Col2 | Cnt
-----+------+----
[no results]
This is because the count is for each combination of Col1 and Col2 (each of which are unique).
Finally, by using either a nested query (as in my answer) or an analytic function (as in #rs.'s answer), you'll get the following result (query changed slightly to return the count):
select t1.col1, t1.col2, cnt
from table1 t1
join (select col1, count(*) as cnt
from table1
group by col1
having count (col1) > 1) t2
on table1.col1 = t2.col1
Col1 | Col2 | Cnt
-----+------+----
1 | A | 2
1 | B | 2
2 | C | 2
2 | D | 2

Answer 3

You should list all selected columns in the group by clause as well.
select col1,
col2,
count(col1)
from table1
group by col1, col2
having count (col1) > 1
order by 2 desc;

Answer 4

Cause of Error
You tried to execute an SQL SELECT statement that included a GROUP BY
function (ie: SQL MIN Function, SQL MAX Function, SQL SUM Function,
SQL COUNT Function) and an expression in the SELECT list that was not
in the SQL GROUP BY clause.
select col1,
col2,
count(col1)
from table1
group by col1,col2
having count (col1) > 1
order by 2 desc;