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Mysql Alter Column

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Mysql Alter Column is used to redefine the table and change the datatype of existing column.

Understand with Example

The Tutorial illustrate an example from 'Mysql Alter Column'. To understand an example from 'Mysql Alter Column', we create a table 'employee1' with required fieldname and datatype.

Query to create table:

mysql>CREATE TABLE employee1 (              
         ->    Empid int(10),         
         ->    Empname varchar(60),   
         ->    Date date 
         ->  );

Query to insert data into Table named employee1:

To insert the records or rows into the table 'employee1' we use insert into query  that adds the records or rows into the table 'employee1'.

mysql>insert into employee1 values(01,'Girish','2008-12-20');
Query OK, 1 row affected (0.02 sec)
mysql>insert into employee1 values(02,'Komal','2008-12-21');
Query OK, 1 row affected (0.02 sec)
mysql>insert into employee1 values(03,'vineet','2008-12-21');
Query OK, 1 row affected (0.02 sec)
mysql>insert into employee1 values(04,'Amit','2008-12-20');
Query OK, 1 row affected (0.02 sec)
mysql>insert into employee1 values(02,'Komal','2008-12-23');
Query OK, 1 row affected (0.02 sec)
mysql>insert into employee1 values(02,'Sandeep','2008-12-24');
Query OK, 1 row affected (0.02 sec)
mysql>insert into employee1 values(02,'suman','2008-12-25');
Query OK, 1 row affected (0.02 sec)
mysql>insert into employee1 values(01,'AAA',' 2008-12-03');
Query OK, 1 row affected (0.02 sec)

Query to view data of  Table named employee1:

To view the detail of  table 'employee1', we  use select query that return the records. 

mysql> select * from employee1;

Output:

+-------+---------+------------+
| Empid | Empname | Date       |
+-------+---------+------------+
|     1 | Girish  | 2008-12-20 |
|     2 | Komal   | 2008-12-21 |
|     3 | vineet  | 2008-12-21 |
|     4 | Amit    | 2008-12-20 |
|     2 | Komal   | 2008-12-23 |
|     2 | Sandeep | 2008-12-24 |
|     2 | suman   | 2008-12-25 |
|     1 | AAA     | 2008-12-03 |
+-------+---------+------------+
8 rows in set (0.00 sec)
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Query to view datatype of  Table named employee1:

The Query describe returns you the table 'employee1'  properties like Fieldname, Type, Null, etc.  

mysql> describe employee1;

Output:

+-------------+-------------+------+-----+---------+-------+
| Field       | Type        | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| Empid       | int(10)     | YES  |     | NULL    |       |
| Empname     | varchar(60) | YES  |     | NULL    |       |
| Dateofbirth | date        | YES  |     | NULL    |       |
+-------------+-------------+------+-----+---------+-------+
3 rows in set (0.02 sec)

Query for altering the column type of the table employee1:

The Query Alter keyword in Mysql is used to redefine the table 'employee1'. The change keyword  changes the datatype of existing column in table.It return you the datatype ' varchar'  from' integer' type.

mysql> alter table employee1
    -> change Empid Empid varchar(30);
Query OK, 8 rows affected (0.17 sec)
Records: 8  Duplicates: 0  Warnings: 0

Output:

 When you  view the table employee1, the describe keyword return you the modified definition table'employee1' .

mysql> describe employee1;
+-------------+-------------+------+-----+---------+-------+
| Field       | Type        | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| Empid       | varchar(30) | YES  |     | NULL    |       |
| Empname     | varchar(60) | YES  |     | NULL    |       |
| Dateofbirth | date        | YES  |     | NULL    |       |
+-------------+-------------+------+-----+---------+-------+
3 rows in set (0.03 sec)
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