SQL: ALTER TABLE Statement

The SQL 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.

SQL ALTER TABLE - Renaming a table

To rename a table, the SQL ALTER TABLE syntax 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.

SQL ALTER TABLE - Adding column(s) to a table

Syntax #1

To add a column to an existing table, the SQL 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 SQL 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.

SQL ALTER TABLE - Modifying column(s) in a table

Syntax #1

To modify a column in an existing table, the SQL 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 SQL 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.

SQL ALTER TABLE - Drop column(s) in a table

Syntax #1

To drop a column in an existing table, the SQL 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.

SQL ALTER TABLE - Rename column(s) in a table
(NEW in Oracle 9i Release 2)

Syntax #1

Starting in Oracle 9i Release 2, you can now rename a column.

To rename a column in an existing table, the SQL 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.

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 SQL 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 SQL 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 SQL 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 SQL 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 SQL 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 SQL 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 SQL 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;