In a relational database management system, the `ALTER TABLE` statement is an SQL command used to modify the structure or definition of an existing table without deleting the table itself. After a table has been created using the `CREATE TABLE` statement, requirements may change. A database administrator or developer may need to add a new column, change the definition of an existing column, rename a column, remove a column, or add or remove constraints. The `ALTER TABLE` command provides a convenient way to make such structural changes while preserving the existing table and, in most cases, its stored data. The basic syntax of the `ALTER TABLE` command is: ```sql ALTER TABLE table_name action; ``` Here, `table_name` represents the name of the table that needs to be modified, while `action` specifies the particular structural change to be performed. The exact syntax of the action may vary slightly between database management systems such as MySQL, PostgreSQL, Oracle, and SQL Server. One of the most common uses of `ALTER TABLE` is to add a new column to an existing table. Suppose there is a table called `Student` containing the columns `Student_ID`, `Name`, and `Course`. If it becomes necessary to store the student's email address, a new column can be added using: ```sql ALTER TABLE Student ADD Email VARCHAR(100); ``` After executing this statement, the `Student` table will contain the additional `Email` column. Existing records remain in the table, although the new column will generally contain `NULL` values for existing rows unless a default value has been specified. Multiple columns can also be added, depending on the SQL implementation. For example: ```sql ALTER TABLE Student ADD Phone VARCHAR(15), ADD Address VARCHAR(200); ``` Adding columns is useful when new information needs to be maintained as part of an existing database design. It avoids the need to create an entirely new table merely because additional attributes are required. Another important use of `ALTER TABLE` is changing the data type or definition of an existing column. For example, suppose the `Name` column was initially defined with a length of 30 characters, but the organization now requires names of up to 100 characters. The column can be modified using a statement such as: ```sql ALTER TABLE Student MODIFY Name VARCHAR(100); ``` The `MODIFY` keyword is commonly used in MySQL and Oracle, although the exact syntax differs across database systems. In SQL Server, for example, `ALTER COLUMN` is used: ```sql ALTER TABLE Student ALTER COLUMN Name VARCHAR(100); ```