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