Find and display duplicate values only once from a column in MySQL

Let us first create a table −

mysql> create table DemoTable
-> (
-> value int
-> );
Query OK, 0 rows affected (0.82 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable values(10);
Query OK, 1 row affected (0.19 sec)

mysql> insert into DemoTable values(100);
Query OK, 1 row affected (0.10 sec)

mysql> insert into DemoTable values(20);
Query OK, 1 row affected (0.28 sec)

mysql> insert into DemoTable values(10);
Query OK, 1 row affected (0.34 sec)

mysql> insert into DemoTable values(30);
Query OK, 1 row affected (0.24 sec)

mysql> insert into DemoTable values(40);
Query OK, 1 row affected (0.11 sec)

mysql> insert into DemoTable values(20);
Query OK, 1 row affected (0.07 sec)

Display all records from the table using select statement −

mysql> select *from DemoTable;

Output

This will produce the following output −

+-------+
| value |
+-------+
| 10    |
| 100   |
| 20    |
| 10    |
| 30    | 
| 40    |
| 20    |
+-------+
7 rows in set (0.00 sec)

Following is the query to find the duplicate values and display them once −

mysql> select *from DemoTable group by value having count(*) > 1;

Output

This will produce the following output −

+-------+
| value |
+-------+
| 10    |
| 20    |
+-------+
2 rows in set (0.00 sec)