How to find duplicate values in a table column?
Posted by admin | Under SQL Tuesday Jan 10, 2012There was an error with one of our application, and I needed to find table rows with duplicate values. The table column was not defined to have a unique index, but we weren’t anticipating duplicate values. I needed an easier way to identify duplicate values in a table with simple SQL statement.
Consider the following “employee” table:
| id | Name | Alias | Age |
|---|---|---|---|
| 1 | John Doe | John | 30 |
| 2 | John Smith | John | 40 |
| 3 | Joe Schmo | Joe | 38 |
| 4 | Charlie Bohne | Charlie | 55 |
Assuming that we have an “employee” table with above values, and looking to find records with duplicate “Alias”. How do we retrieve them? With the following SQL statement with HAVING clause, we can easily accomplish that.
SELECT alias, count(alias) as count
FROM employee
GROUP BY alias
HAVING (count(alias) > 1)
ORDER BY alias
The above SQL statement will retrieve:
| Alias | Count |
|---|---|
| John | 2 |
*NOTE: The HAVING clause allows SQL to use with aggregate functions with a condition whereas WHERE clause does not offer that functionality. For example, the HAVING clause can be used to retrieve SUM(x) > 100 or COUNT(y) > 1.
