Unique Keys: A unique index allows only unique values in the column. : Unique « Table « MySQL Tutorial

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MySQL Tutorial » Table » Unique 
4.17.1.Unique Keys: A unique index allows only unique values in the column.
mysql>
mysql>
mysql>
mysql>
mysql> CREATE TABLE Employee(
    ->     id            int,
    ->     first_name    VARCHAR(15),
    ->     last_name     VARCHAR(15),
    ->     start_date    DATE,
    ->     end_date      DATE,
    ->     salary        FLOAT(8,2),
    ->     city          VARCHAR(10),
    ->     description   VARCHAR(15)
    -> );
Query OK, rows affected (0.03 sec)

mysql>
mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->              values (1,'Jason',    'Martin',  '19960725',  '20060725', 1234.56'Toronto',  'Programmer');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(2,'Alison',   'Mathews',  '19760321', '19860221', 6661.78'Vancouver','Tester');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(3,'James',    'Smith',    '19781212', '19900315', 6544.78'Vancouver','Tester');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(4,'Celia',    'Rice',     '19821024', '19990421', 2344.78'Vancouver','Manager');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(5,'Robert',   'Black',    '19840115', '19980808', 2334.78'Vancouver','Tester');
Query OK, row affected (0.02 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(6,'Linda',    'Green',    '19870730', '19960104', 4322.78,'New York',  'Tester');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(7,'David',    'Larry',    '19901231', '19980212', 7897.78,'New York',  'Manager');
Query OK, row affected (0.00 sec)

mysql>
mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
    ->               values(8,'James',    'Cat',     '19960917',  '20020415', 1232.78,'Vancouver', 'Tester');
Query OK, row affected (0.00 sec)

mysql>
mysql> select from Employee;
+------+------------+-----------+------------+------------+---------+-----------+-------------+
| id   | first_name | last_name | start_date | end_date   | salary  | city      | description |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
|    | Jason      | Martin    | 1996-07-25 2006-07-25 1234.56 | Toronto   | Programmer  |
|    | Alison     | Mathews   | 1976-03-21 1986-02-21 6661.78 | Vancouver | Tester      |
|    | James      | Smith     | 1978-12-12 1990-03-15 6544.78 | Vancouver | Tester      |
|    | Celia      | Rice      | 1982-10-24 1999-04-21 2344.78 | Vancouver | Manager     |
|    | Robert     | Black     | 1984-01-15 1998-08-08 2334.78 | Vancouver | Tester      |
|    | Linda      | Green     | 1987-07-30 1996-01-04 4322.78 | New York  | Tester      |
|    | David      | Larry     | 1990-12-31 1998-02-12 7897.78 | New York  | Manager     |
|    | James      | Cat       | 1996-09-17 2002-04-15 1232.78 | Vancouver | Tester      |
+------+------------+-----------+------------+------------+---------+-----------+-------------+
rows in set (0.00 sec)

mysql>
mysql>
mysql> ALTER TABLE Employee ADD UNIQUE lastn_index (last_name);
Query OK, rows affected (0.03 sec)
Records: 8  Duplicates: 0  Warnings: 0

mysql>
mysql>
mysql> drop table Employee;
Query OK, rows affected (0.00 sec)

mysql>
mysql>
mysql>
4.17.Unique
4.17.1.Unique Keys: A unique index allows only unique values in the column.
4.17.2.You can create more than one unique key.
4.17.3.Add Unique constaints to cover two columns
4.17.4.ALTER TABLE table_name ADD UNIQUE index_name (column_name[,...]);
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