Oracle Database Interview Question:

How To Count Duplicated Values in a Column in Oracle?

Submitted by: Administrator

If you have a column with duplicated values, and you want to know what are those duplicated values are and how many duplicates are there for each of those values, you can use the GROUP BY ... HAVING clause as shown in the following example. It returns how many duplicated first names in the employees table:

SQL> SELECT first_name, COUNT(*) FROM employees
GROUP BY first_name HAVING COUNT(*) > 1;
<pre>FIRST_NAME COUNT(*)
-------------------- ----------
Peter 3
Michael 2
Steven 2
John 3
Julia 2
William 2
Karen 2
Kevin 2
......</pre>
Submitted by: Administrator