Check for Duplicates in MySQL Table Over Multiple Columns

To check for duplicates in MySQL, you can use group by having clause. The syntax is as follows.

select yourColumnName1,yourColumnName2,......N,count(*) as anyVariableName from
yourTableName
group by yourColumnName1,yourColumnName2
having count(*) > 1;

To understand the above syntax, let us create a table. The query to create a table is as follows.

mysql> create table DuplicateDemo
-> (
-> StudentId int not null,
-> StudentFirstName varchar(100),
-> StudentLastName varchar(100),
-> Primary Key(StudentId)
-> );
Query OK, 0 rows affected (0.50 sec)

Insert some records in the table using insert command. The query is as follows.

mysql> insert into DuplicateDemo values(1,'John','Smith');
Query OK, 1 row affected (0.13 sec)

mysql> insert into DuplicateDemo values(2,'Mike','Jones');
Query OK, 1 row affected (0.28 sec)

mysql> insert into DuplicateDemo values(3,'David','Smith');
Query OK, 1 row affected (0.15 sec)

mysql> insert into DuplicateDemo values(4,'Carol','Taylor');
Query OK, 1 row affected (0.20 sec)

mysql> insert into DuplicateDemo values(5,'David','Smith');
Query OK, 1 row affected (0.11 sec)

mysql> insert into DuplicateDemo values(6,'John','Smith');
Query OK, 1 row affected (0.16 sec)

mysql> insert into DuplicateDemo values(7,'John','Taylor');
Query OK, 1 row affected (0.15 sec)

Display all records from the table using select statement.

The query is as follows -

mysql> select *from DuplicateDemo;

The following is the output.

+-----------+------------------+-----------------+
| StudentId | StudentFirstName | StudentLastName |
+-----------+------------------+-----------------+
| 1         | John             | Smith           |
| 2         | Mike             | Jones           |
| 3         | David            | Smith           |
| 4         | Carol            | Taylor          |
| 5         | David            | Smith           |
| 6         | John             | Smith           |
| 7         | John             | Taylor          |
+-----------+------------------+-----------------+
7 rows in set (0.00 sec)

Here is the query to check the duplicates from the table.

mysql> select StudentFirstName,StudentLastName,count(*) as Total from DuplicateDemo
-> group by StudentFirstName,StudentLastName
-> having count(*) > 1;

The following is the output.

+------------------+-----------------+-------+
| StudentFirstName | StudentLastName | Total |
+------------------+-----------------+-------+
| John             | Smith           | 2     |
| David            | Smith           | 2     |
+------------------+-----------------+-------+
2 rows in set (0.00 sec)