How to prevent MySQL double insert (duplicate entry)?

To prevent duplicate entry, add constraint UNIQUE. Let us first create a table −

mysql> create table DemoTable
(
   Id int,
   Name varchar(100)
);
Query OK, 0 rows affected (0.79 sec)

Here is the query to prevent MySQL double insert using UNIQUE −

mysql> alter table DemoTable add constraint id_NameUnKey UNIQUE(Id,Name);
Query OK, 0 rows affected (0.82 sec)
Records: 0 Duplicates: 0 Warnings: 0

Insert records in the table using insert command. When we will try to the same record again, then the “Duplicate entry” error will be visible −

mysql> insert into DemoTable values(11,'John');
Query OK, 1 row affected (0.18 sec)
mysql> insert into DemoTable values(12,'John');
Query OK, 1 row affected (0.21 sec)
mysql> insert into DemoTable values(11,'John');
ERROR 1062 (23000): Duplicate entry '11-John' for key 'id_NameUnKey'

Display records from the table using select command −

mysql> select *from DemoTable;

This will produce the following output −

+------+------+
| Id   | Name |
+------+------+
| 11   | John |
| 12   | John |
+------+------+
2 rows in set (0.00 sec)