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.

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.

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.

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.

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.

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.

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.