Oracle Topics

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.