MySQL select only duplicate records from database and display the count as well?

To select only duplicate records from database and display the count, use HAVING along with aggregate function count(). Let us first create a table −

mysql> create table duplicateRecords
   -> (
   -> ClientId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   -> ClientName varchar(20)
   -> );
Query OK, 0 rows affected (0.49 sec)

Following is the query to insert records in the table using insert command −

mysql> insert into duplicateRecords(ClientName) values('John');
Query OK, 1 row affected (0.16 sec)
mysql> insert into duplicateRecords(ClientName) values('Carol');
Query OK, 1 row affected (0.17 sec)
mysql> insert into duplicateRecords(ClientName) values('John');
Query OK, 1 row affected (0.29 sec)
mysql> insert into duplicateRecords(ClientName) values('Sam');
Query OK, 1 row affected (0.19 sec)
mysql> insert into duplicateRecords(ClientName) values('Sam');
Query OK, 1 row affected (0.11 sec)
mysql> insert into duplicateRecords(ClientName) values('Bob');
Query OK, 1 row affected (0.12 sec)
mysql> insert into duplicateRecords(ClientName) values('John');
Query OK, 1 row affected (0.13 sec)
mysql> insert into duplicateRecords(ClientName) values('Sam');
Query OK, 1 row affected (0.12 sec)

Following is the query to display all records from the table using select statement −

mysql> select * from duplicateRecords;

This will produce the following output −

+----------+------------+
| ClientId | ClientName |
+----------+------------+
| 1        | John       |
| 2        | Carol      |
| 3        | John       |
| 4        | Sam        |
| 5        | Sam        |
| 6        | Bob        |
| 7        | John       |
| 8        | Sam        |
+----------+------------+
8 rows in set (0.00 sec)

Following is the query:to select only duplicate records from database −

mysql> select ClientName,count(*) as DuplicateRecord
   -> from duplicateRecords
   -> group by ClientName
   -> having DuplicateRecord > 1;

This will produce the following output −

+------------+-----------------+
| ClientName | DuplicateRecord |
+------------+-----------------+
| John       | 3               |
| Sam        | 3               |
+------------+-----------------+
2 rows in set (0.00 sec)