Oracle ALTER TABLE Statement

In Oracle, ALTER TABLE statement specifies how to add, modify, drop or delete columns in a table. It is also used to rename a table.

How to add column in a table

Syntax:

ALTER TABLE table_name  

  ADD column_name column-definition;   

Example:

Consider that already existing table customers. Now, add a new column customer_age into the table customers.

ALTER TABLE customers  

  ADD customer_age varchar2(50);  

Now, a new column “customer_age” will be added in customers table.

How to add multiple columns in the existing table

Syntax:

ALTER TABLE table_name  

  ADD (column_1 column-definition,  

       column_2 column-definition,  

       ...  

       column_n column_definition); 

Example

ALTER TABLE customers  

  ADD (customer_type varchar2(50),  

       customer_address varchar2(50)); 

Now, two columns customer_type and customer_address will be added in the table customers.

How to modify column of a table

Syntax:

ALTER TABLE table_name  

  MODIFY column_name column_type; 

Example:

ALTER TABLE customers  

  MODIFY customer_name varchar2(100) not null; 

Now the column column_name in the customers table is modified
to varchar2 (100) and forced the column to not allow null values. 

How to modify multiple columns of a table

Syntax:

ALTER TABLE table_name  



  MODIFY (column_1 column_type,  



          column_2 column_type,  



          ...  



          column_n column_type);  

Example:

ALTER TABLE customers  

  MODIFY (customer_name varchar2(100) not null,  

          city varchar2(100));

This will modify both the customer_name and city columns in the table. 

How to drop column of a table

Syntax:

ALTER TABLE table_name  

  DROP COLUMN column_name;

Example:

ALTER TABLE customers  

  DROP COLUMN customer_name; 

This will drop the customer_name column from the table.

How to rename column of a table

Syntax:

ALTER TABLE table_name  

  RENAME COLUMN old_name to new_name; 

Example:

ALTER TABLE customers  

 RENAME COLUMN customer_name to cname; 

This will rename the column customer_name into cname.

How to rename table

Syntax:

ALTER TABLE table_name  

  RENAME TO new_table_name;  

Example:

 ALTER TABLE customers  

RENAME TO retailers;

This will rename the customer table into "retailers" table.