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;