Select all duplicate MySQL rows based on one or two columns?

For this, use subquery along with HAVING clause. Let us first create a table −

mysql> create table DemoTable
   (
   StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   StudentFirstName varchar(20),
   StudentLastName varchar(20)
   );
Query OK, 0 rows affected (0.27 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('John','Smith');
Query OK, 1 row affected (0.04 sec)

mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('Carol','Taylor');
Query OK, 1 row affected (0.04 sec)

mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('John','Doe');
Query OK, 1 row affected (0.17 sec)

mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('John','Brown');
Query OK, 1 row affected (0.05 sec)

mysql> insert into DemoTable(StudentFirstName,StudentLastName) values('David','Miller');
Query OK, 1 row affected (0.06 sec)

Display all records from the table using select statement −

mysql> select *from DemoTable;

This will produce the following output −

+-----------+------------------+-----------------+
| StudentId | StudentFirstName | StudentLastName |
+-----------+------------------+-----------------+
| 1         | John             | Smith           |
| 2         | Carol            | Taylor          |
| 3         | John             | Doe             |
| 4         | John             | Brown           |
| 5         | David            | Miller          |
+-----------+------------------+-----------------+
5 rows in set (0.00 sec)

Following is the query to select all duplicate rows based on one or two columns. Here, we are counting the names appearing more than once i.e. duplicates −

mysql> select StudentId from DemoTable
   where StudentFirstName=(select StudentFirstName from DemoTable having count(StudentFirstName) > 1);

This will produce the following output −

+-----------+
| StudentId |
+-----------+
| 1         |
| 3         |
| 4         |
+-----------+
3 rows in set (0.03 sec)