ALTER TABLE Statement


The ALTER TABLE statement allows you to rename an existing table. It can also be used to add, modify, or drop a column from an existing table.

 Renaming a table

The basic syntax for renaming a table is:

ALTER TABLE table_name
RENAME TO new_table_name;

For example:

ALTER TABLE suppliers
RENAME TO vendors;

This will rename the suppliers table to vendors.

Adding column(s) to a table

Syntax #1

To add a column to an existing table, the ALTER TABLE syntax is:

ALTER TABLE table_name
ADD column_name column-definition;

For example:

ALTER TABLE supplier
ADD supplier_name  varchar2(50);

This will add a column called supplier_name to the supplier table.

Syntax #2

To add multiple columns to an existing table, the ALTER TABLE syntax is:

ALTER TABLE table_name
ADD ( column_1 column-definition,
  column_2 column-definition,
   
  column_n column_definition );

For example:

ALTER TABLE supplier
ADD ( supplier_name varchar2(50),
  City varchar2(45) );

This will add two columns (supplier_name and city) to the supplier table.

Modifying column(s) in a table

Syntax #1

To modify a column in an existing table, the ALTER TABLE syntax is:

ALTER TABLE table_name
MODIFY column_name column_type;

For example:

ALTER TABLE supplier
MODIFY supplier_name   varchar2(100)     not null;

This will modify the column called supplier_name to be a data type of varchar2(100) and force the column to not allow null values.

Syntax #2

To modify multiple columns in an existing table, the ALTER TABLE syntax is:

ALTER TABLE table_name
MODIFY ( column_1 column_type,
  column_2 column_type,
   
  column_n column_type );

For example:

ALTER TABLE supplier
MODIFY ( supplier_name varchar2(100) not null,
  city varchar2(75)   );

This will modify both the supplier_name and city columns.

Drop column(s) in a table

Syntax #1

To drop a column in an existing table, the ALTER TABLE syntax is:

ALTER TABLE table_name
DROP COLUMN column_name;

For example:

ALTER TABLE supplier
DROP COLUMN supplier_name;

This will drop the column called supplier_name from the table called supplier.

Rename column(s) in a table
(NEW in Oracle 9i Release 2)

Syntax #1

To rename a column in an existing table, the ALTER TABLE syntax is:

ALTER TABLE table_name
RENAME COLUMN old_name to new_name;

For example:

ALTER TABLE supplier
RENAME COLUMN supplier_name to sname;

Practice Exercise #1:

Based on the departments table below, rename the departments table to depts.

CREATE TABLE departments
( department_id number(10) not null,
  department_name varchar2(50) not null,
  CONSTRAINT departments_pk PRIMARY KEY (department_id)
);      

Solution:

The following ALTER TABLE statement would rename the departments table to depts:

ALTER TABLE departments
RENAME TO depts;

Practice Exercise #2:

Based on the employees table below, add a column called salary that is a number(6) datatype.

CREATE TABLE employees
( employee_number number(10) not null,
  employee_name varchar2(50) not null,
  department_id number(10),  
  CONSTRAINT employees_pk PRIMARY KEY (employee_number)
);      

Solution:

The following ALTER TABLE statement would add a salary column to the employees table:

ALTER TABLE employees
ADD salary number(6);

Practice Exercise #3:

Based on the customers table below, add two columns – one column called contact_name that is a varchar2(50) datatype and one column called last_contacted that is a date datatype.

CREATE TABLE customers
( customer_id number(10) not null,
  customer_name varchar2(50) not null,
  address varchar2(50),  
  city varchar2(50),  
  state varchar2(25),  
  zip_code varchar2(10),  
  CONSTRAINT customers_pk PRIMARY KEY (customer_id)
);      

Solution:

The following ALTER TABLE statement would add the contact_name and last_contacted columns to the customers table:

ALTER TABLE customers
ADD ( contact_name varchar2(50),
  last_contacted date );

Practice Exercise #4:

Based on the employees table below, change the employee_name column to a varchar2(75) datatype.

CREATE TABLE employees
( employee_number number(10) not null,
  employee_name varchar2(50) not null,
  department_id number(10),  
  CONSTRAINT employees_pk PRIMARY KEY (employee_number)
);      

Solution:

The following ALTER TABLE statement would change the datatype for the employee_name column to varchar2(75):

ALTER TABLE employees
MODIFY employee_name varchar2(75);

Practice Exercise #5:

Based on the customers table below, change the customer_name column to NOT allow null values and change the state column to a varchar2(2) datatype.

CREATE TABLE customers
( customer_id number(10) not null,
  customer_name varchar2(50),  
  address varchar2(50),  
  city varchar2(50),  
  state varchar2(25),  
  zip_code varchar2(10),  
  CONSTRAINT customers_pk PRIMARY KEY (customer_id)
);      

Solution:

The following ALTER TABLE statement would modify the customer_name and state columns accordingly in the customers table:

ALTER TABLE customers
MODIFY ( customer_name varchar2(50) not null,
  state varchar2(2) );

Practice Exercise #6:

Based on the employees table below, drop the salary column.

CREATE TABLE employees
( employee_number number(10) not null,
  employee_name varchar2(50) not null,
  department_id number(10),  
  salary number(6),  
  CONSTRAINT employees_pk PRIMARY KEY (employee_number)
);      

Solution:

The following ALTER TABLE statement would drop the salary column from the employees table:

ALTER TABLE employees
DROP COLUMN salary;

Practice Exercise #7:

Based on the departments table below, rename the department_name column to dept_name.

CREATE TABLE departments
( department_id number(10) not null,
  department_name varchar2(50) not null,
  CONSTRAINT departments_pk PRIMARY KEY (department_id)
);      

Solution:

The following ALTER TABLE statement would rename the department_name column to dept_name in the departments table:

ALTER TABLE departments
RENAME COLUMN department_name to dept_name;

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