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Mysql Alter Unique Key

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Mysql Alter Unique is used to change the table structure and add unique key on specified column of a table.

Mysql Alter Unique Key

     

Mysql Alter Unique is used to change the table structure and add unique key on specified column of a table.It allows one NULL value to be inserted in the column. Table can contains more than one unique keys.

Understand with Example

The Tutorial illustrate an example from 'Mysql Alter Unique Key'. The example create a table 'employee' with required fieldnames and datetype respectively. The table has a ID as Primary Key.

 Create table "employee": 

CREATE TABLE `employee` ( 
`ID` bigint(20)  NOT NULL, 
`EmpID` bigint(20)  NOT NULL, 
PRIMARY KEY (`ID`) 
)

Query to alter the column unique : 

The Query Alter is used to modify the table 'Employee' and add a unique multiple columns to the table 'Employee'. 

mysql> ALTER TABLE Employee ADD UNIQUE (ID, EmpID)
Query OK, 2 rows affected (0.16 sec)

Describe the table "employee" :

The Describe table is used to show the fieldnames, Type, Null, etc. of table employee.

mysql> describe employee;
+-------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| ID | bigint(20) unsigned | NO | PRI | | |
| EmpID | bigint(20) unsigned | NO | | | |
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Posted on: January 16, 2009

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