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SQL : ALTER TABLE Statement

Tutorial by:      Date: 2016-04-14 23:48:02

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This SQL tutorial explains how to use the SQL ALTER TABLE statement to add a column, modify a column, drop a column, rename a column or rename a table (with lots of clear, concise examples). We've also added some practice exercises that you can try for yourself.

Description

The SQL ALTER TABLE statement is used to add, modify, or drop/delete columns in a table. The SQL ALTER TABLE statement is also used to rename a table.

Add column in table

Syntax

To add a column in a table, the SQL ALTER TABLE syntax is:

ALTER TABLE table_name
  ADD column_name column-definition;

Example

Let's look at a SQL ALTER TABLE example that adds a column.

For example:

ALTER TABLE supplier
  ADD supplier_name char(50);

This SQL ALTER TABLE example will add a column called supplier_name to the supplier table.

Add multiple columns in table

Syntax

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);

Example

Let's look at SQL ALTER TABLE example that adds more than one column.

For example:

ALTER TABLE supplier
  ADD (supplier_name char(50),
       city char(45));

This SQL ALTER TABLE example will add two columns, supplier_name as a char(50) field and city as a char(45) field to the supplier table.

Modify column in table

Syntax

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

For Oracle, MySQL, MariaDB:

ALTER TABLE table_name
  MODIFY column_name column_type;

For SQL Server:

ALTER TABLE table_name
  ALTER COLUMN column_name column_type;

For PostgreSQL:

ALTER TABLE table_name
  ALTER COLUMN column_name TYPE column_definition;

Example

Let's look at an example of how to modify a column called supplier_name using the ALTER TABLE statement. Note that most databases have a slightly different syntax.

For Oracle:

ALTER TABLE supplier
  MODIFY supplier_name char(100) NOT NULL;

For MySQL and MariaDB:

ALTER TABLE supplier
  MODIFY supplier_name VARCHAR(100) NOT NULL;

For SQL Server:

ALTER TABLE supplier
  ALTER COLUMN supplier_name VARCHAR(100) NOT NULL;

For PostgreSQL:

ALTER TABLE supplier
  ALTER COLUMN supplier_name TYPE CHAR(100),
  ALTER COLUMN supplier_name SET NOT NULL;

Modify multiple columns in table

Syntax

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

For Oracle:

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

For MySQL and MariaDB:

ALTER TABLE table_name
  MODIFY column_1 column_definition
    [ FIRST | AFTER column_name ],
  MODIFY column_2 column_definition
    [ FIRST | AFTER column_name ],
  ...
;

For PostgreSQL:

ALTER TABLE table_name
  ALTER COLUMN column_name TYPE column_definition,
  ALTER COLUMN column_name TYPE column_definition,
  ...
;

Example

Let's look at an example that uses the ALTER TABLE statement to modify more than one column. In this example, we will modify two columns called supplier_name and city.

For Oracle:

ALTER TABLE supplier
  MODIFY (supplier_name char(100) NOT NULL,
          city char(75));

For MySQL and MariaDB:

ALTER TABLE supplier
  MODIFY supplier_name VARCHAR(100) NOT NULL,
  MODIFY city VARCHAR(75);

For PostgreSQL:

ALTER TABLE supplier
  ALTER COLUMN supplier_name TYPE CHAR(100),
  ALTER COLUMN supplier_name SET NOT NULL,
  ALTER COLUMN city TYPE CHAR(75);

Drop column in table

Syntax

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

ALTER TABLE table_name
  DROP COLUMN column_name;

Example

Let's look at an example that drops (ie: deletes) a column from a table.

For example:

ALTER TABLE supplier
  DROP COLUMN supplier_name;

This SQL ALTER TABLE example will drop the column called supplier_name from the table called supplier.

Rename column in table

Syntax

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

For Oracle and PostgreSQL:

ALTER TABLE table_name
  RENAME COLUMN old_name TO new_name;

For SQL Server (using the stored procedure called sp_rename):

sp_rename 'table_name.old_column', 'new_name', 'COLUMN';

For MySQL and MariaDB:

ALTER TABLE table_name
  CHANGE COLUMN old_name TO new_name;

Example

Let's look at an example that renames a column in the supplier table from supplier_name to sname.

For Oracle (9i Rel2 and up) and PostgreSQL:

ALTER TABLE supplier
  RENAME COLUMN supplier_name TO sname;

For SQL Server (using the stored procedure called sp_rename):

sp_rename 'supplier.supplier_name', 'sname', 'COLUMN';

For MySQL and MariaDB:

ALTER TABLE supplier
  CHANGE COLUMN supplier_name sname VARCHAR(100);

In MySQL and MariaDB, you must specify the data type of the column when you rename it.

Rename table

Syntax

To rename a table, the SQL ALTER TABLE syntax is:

For Oracle, MySQL, MariaDB, PostgreSQL and SQLite:

ALTER TABLE table_name
  RENAME TO new_table_name;

For SQL Server (using the stored procedure called sp_rename):

sp_rename 'table_name', 'new_table_name';

Example

Let's look at an example that renames a table called supplier to the new name vendor.

For Oracle, MySQL, MariaDB, PostgreSQL and SQLite:

ALTER TABLE supplier
  RENAME TO vendor;

For SQL Server (using the stored procedure called sp_rename):

sp_rename 'supplier', 'vendor';

Practice Exercise #1:

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

CREATE TABLE departments
( department_id int NOT NULL,
  department_name char(50) NOT NULL,
  CONSTRAINT departments_pk PRIMARY KEY (department_id)
);

Solution for Practice Exercise #1:

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 an int datatype.

CREATE TABLE employees
( employee_number int NOT NULL,
  employee_name char(50) NOT NULL,
  department_id int,
  CONSTRAINT employees_pk PRIMARY KEY (employee_number)
);

Solution for Practice Exercise #2:

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

ALTER TABLE employees
  ADD salary int;

Practice Exercise #3:

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

CREATE TABLE customers
( customer_id int NOT NULL,
  customer_name char(50) NOT NULL,
  address char(50),
  city char(50),
  state char(25),
  zip_code char(10),
  CONSTRAINT customers_pk PRIMARY KEY (customer_id)
);

Solution for Practice Exercise #3:

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 char(50),
       last_contacted date);

Practice Exercise #4:

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

CREATE TABLE employees
( employee_number int NOT NULL,
  employee_name char(50) NOT NULL,
  department_id int,
  CONSTRAINT employees_pk PRIMARY KEY (employee_number)
);

Solution for Practice Exercise #4:

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

ALTER TABLE employees
  MODIFY employee_name char(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 char(2) datatype.

CREATE TABLE customers
( customer_id int NOT NULL,
  customer_name char(50),
  address char(50),
  city char(50),
  state char(25),
  zip_code char(10),
  CONSTRAINT customers_pk PRIMARY KEY (customer_id)
);

Solution for Practice Exercise #5:

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 char(50) NOT NULL,
          state char(2));

Practice Exercise #6:

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

CREATE TABLE employees
( employee_number int NOT NULL,
  employee_name char(50) NOT NULL,
  department_id int,
  salary int,
  CONSTRAINT employees_pk PRIMARY KEY (employee_number)
);

Solution for Practice Exercise #6:

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 int NOT NULL,
  department_name char(50) NOT NULL,
  CONSTRAINT departments_pk PRIMARY KEY (department_id)
);

Solution for Practice Exercise #7:

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;

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