What does ALTER do in SQL?

ALTER is a fundamental command in SQL (Structured Query Language) that allows users to modify the structure of database objects such as tables, views, and indexes. It is an essential tool for database administrators and developers to adapt their database schema to changing requirements or to correct errors in the existing structure. In this article, we will explore the various operations that can be performed using the ALTER command in SQL.

Modifying Table Structure

One of the primary uses of the ALTER command is to modify the structure of a table. This can include adding or dropping columns, modifying the data type of existing columns, or renaming columns. Here are some examples of ALTER TABLE operations:

– Adding a Column: To add a new column to an existing table, you can use the following syntax:
“`sql
ALTER TABLE table_name ADD column_name data_type;
“`
For instance, to add a new column named “age” of type INT to a table named “employees,” you would execute:
“`sql
ALTER TABLE employees ADD age INT;
“`

– Dropping a Column: To remove a column from a table, use the DROP COLUMN clause:
“`sql
ALTER TABLE table_name DROP COLUMN column_name;
“`
For example, to drop the “age” column from the “employees” table, you would execute:
“`sql
ALTER TABLE employees DROP COLUMN age;
“`

– Modifying Column Data Type: To change the data type of an existing column, use the ALTER COLUMN clause:
“`sql
ALTER TABLE table_name ALTER COLUMN column_name new_data_type;
“`
For example, to change the data type of the “salary” column in the “employees” table from DECIMAL to FLOAT, you would execute:
“`sql
ALTER TABLE employees ALTER COLUMN salary FLOAT;
“`

– Renaming a Column: To rename a column, use the RENAME COLUMN clause:
“`sql
ALTER TABLE table_name RENAME COLUMN old_column_name TO new_column_name;
“`
For instance, to rename the “age” column in the “employees” table to “years_old,” you would execute:
“`sql
ALTER TABLE employees RENAME COLUMN age TO years_old;
“`

Creating and Dropping Views

ALTER can also be used to create and drop views in SQL. A view is a virtual table derived from one or more tables, and it can be used to simplify complex queries or to provide a restricted view of the data.

– Creating a View: To create a view, use the CREATE VIEW clause:
“`sql
CREATE VIEW view_name AS
SELECT column1, column2, …
FROM table_name
WHERE condition;
“`
For example, to create a view named “employee_details” that displays the “first_name,” “last_name,” and “salary” columns from the “employees” table, you would execute:
“`sql
CREATE VIEW employee_details AS
SELECT first_name, last_name, salary
FROM employees;
“`

– Dropping a View: To remove a view, use the DROP VIEW clause:
“`sql
DROP VIEW view_name;
“`
For instance, to drop the “employee_details” view, you would execute:
“`sql
DROP VIEW employee_details;
“`

Modifying Indexes

ALTER can also be used to modify indexes on tables. Indexes improve the performance of queries by allowing the database engine to quickly locate the data.

– Adding an Index: To add an index to a table, use the CREATE INDEX clause:
“`sql
CREATE INDEX index_name ON table_name(column_name);
“`
For example, to create an index named “idx_employee_id” on the “employees” table based on the “employee_id” column, you would execute:
“`sql
CREATE INDEX idx_employee_id ON employees(employee_id);
“`

– Dropping an Index: To remove an index from a table, use the DROP INDEX clause:
“`sql
DROP INDEX index_name;
“`
For instance, to drop the “idx_employee_id” index from the “employees” table, you would execute:
“`sql
DROP INDEX idx_employee_id;
“`

In conclusion, the ALTER command in SQL is a versatile tool that allows users to modify the structure of database objects. By understanding the various operations that can be performed using ALTER, you can effectively manage your database schema and ensure that it meets your evolving needs.