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Making an existing field Unique in MySQL?
Unique in MySQL means we cannot add duplicate records. Let us now see how to create a unique constraint in the column at the time of creating a table.
mysql> create table UniqueConstDemo - > ( - > name varchar(100) unique - > ); Query OK, 0 rows affected (0.72 sec)
Now, we cannot have same value more than once for column ‘name’.
Inserting some records with duplicate values to check the error.
mysql> insert into UniqueConstDemo values('John');
Query OK, 1 row affected (0.19 sec)
mysql> insert into UniqueConstDemo values('John');On running the above query, the following error is visible.
mysql> insert into UniqueConstDemo values('John');
ERROR 1062 (23000): Duplicate entry 'John' for key 'name'Inserting different values won’t give an error.
mysql> insert into UniqueConstDemo values('Bob');
Query OK, 1 row affected (0.11 sec)Now, let us display all the 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)
Updated on: 26-Jun-2020
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