Make existing field unique in MySQL?

Uniqueness in MySQL means that we cannot add duplicate records. Now let us see how to create a unique constraint in a column while creating a table.

mysql> create table UniqueConstDemo
- > (
- > name varchar(100) unique
- > );
Query OK, 0 rows affected (0.72 sec)

Now, we cannot use the same value multiple times in the "name" column.

Insert some records with duplicate values ​​to check for errors.

mysql> insert into UniqueConstDemo values('John');
Query OK, 1 row affected (0.19 sec)

mysql> insert into UniqueConstDemo values('John');

When running the above query, the following error occurs.

mysql> insert into UniqueConstDemo values('John');
ERROR 1062 (23000): Duplicate entry 'John' for key 'name'

No errors in inserting different values.

mysql> insert into UniqueConstDemo values('Bob');
Query OK, 1 row affected (0.11 sec)

Now, let us display all records with the help of SELECT statement.

mysql> select *from UniqueConstDemo;

The following is the output.

+-------+
| name  |
+-------+
| Bob   |
| John  |
+-------+
3 rows in set (0.00 sec)

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