SQL: ALTER TABLE Statement

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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

Syntax #1
Starting in Oracle 9i Release 2, you can now rename a column.
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;
This will rename the column called supplier_name to sname.
Acknowledgements: Thanks to Dave M., Craig A., and Susan W. for contributing to this solution!

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;