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alter command

alter command

alter command is used for alteration of table structures. There are various uses of alter command, such as,

  • to add a column to existing table
  • to rename any existing column
  • to change datatype of any column or to modify its size.
  • alter is also used to drop a column.

To Add Column to existing Table

Using alter command we can add a column to an existing table. Following is the Syntax,

alter table table-name add(column-name datatype);

Here is an Example for this,

alter table Student add(address char); 

The above command will add a new column address to the Student table


To Add Multiple Column to existing Table

Using alter command we can even add multiple columns to an existing table. Following is the Syntax,

alter table table-name add(column-name1 datatype1, column-name2 datatype2, column-name3 datatype3);

Here is an Example for this,

alter table Student add(father-name varchar(60), mother-name varchar(60), dob date); 

The above command will add three new columns to the Student table


To Add column with Default Value

alter command can add a new column to an existing table with default values. Following is the Syntax,

alter table table-name add(column-name1 datatype1 default data);

Here is an Example for this,

alter table Student add(dob date default '1-Jan-99'); 

The above command will add a new column with default value to the Student table


To Modify an existing Column

alter command is used to modify data type of an existing column . Following is the Syntax,

alter table table-name modify(column-name datatype);

Here is an Example for this,

alter table Student modify(address varchar(30)); 

The above command will modify address column of the Student table


To Rename a column

Using alter command you can rename an existing column. Following is the Syntax,

alter table table-name rename old-column-name to column-name;

Here is an Example for this,

alter table Student rename address to Location; 

The above command will rename address column to Location.


To Drop a Column

alter command is also used to drop columns also. Following is the Syntax,

alter table table-name drop(column-name);

Here is an Example for this,

alter table Student drop(address);