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Adding unique constraint to ALTER TABLE in MySQL
Let us first create a table −
mysql> create table DemoTable1811 ( FirstName varchar(20), LastName varchar(20) ); Query OK, 0 rows affected (0.00 sec)
Here is the query to add index
mysql> alter table DemoTable1811 ADD UNIQUE unique_index_first_last_name(FirstName, LastName); Query OK, 0 rows affected (0.00 sec) Records: 0 Duplicates: 0 Warnings: 0
Insert some records in the table using insert command −
mysql> insert into DemoTable1811 values('John','Smith');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1811 values('John','Doe');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1811 values('Adam','Smith');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1811 values('John','Doe');
ERROR 1062 (23000): Duplicate entry 'John-Doe' for key 'unique_index_first_last_name'Display all records from the table using select statement −
mysql> select * from DemoTable1811;
This will produce the following output −
+-----------+----------+ | FirstName | LastName | +-----------+----------+ | Adam | Smith | | John | Doe | | John | Smith | +-----------+----------+ 3 rows in set (0.00 sec)
Updated on: 24-Dec-2019
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