The SQL ALTER statement is used to modify the structure of existing database objects such as tables, views, procedures, functions, and triggers. The ALTER statement allows you to change the properties of an existing object without having to drop and recreate it.
Syntax
The syntax for the ALTER statement varies depending on the type of object you want to modify. Here are some examples of how to use the ALTER statement to modify different types of database objects:
ALTER TABLE
To modify a table in SQL, you can use the ALTER TABLE statement followed by the name of the table and the action you want to perform. For example, to add a new column to an existing table, you can use the following syntax:
ALTER TABLE table_name ADD column_name data_type [NULL | NOT NULL] [DEFAULT default_value]
You can also use the ALTER TABLE statement to modify existing columns, rename columns, and change the data type of columns.
ALTER VIEW
To modify a view in SQL, you can use the ALTER VIEW statement followed by the name of the view and the action you want to perform. For example, to modify the SELECT statement of an existing view, you can use the following syntax:
ALTER VIEW view_name AS SELECT ...
You can also use the ALTER VIEW statement to modify the schema binding of a view, rename a view, and add or drop columns from a view.
ALTER PROCEDURE
To modify a stored procedure in SQL, you can use the ALTER PROCEDURE statement followed by the name of the stored procedure and the action you want to perform. For example, to add a new parameter to an existing stored procedure, you can use the following syntax:
ALTER PROCEDURE procedure_name @new_parameter data_type AS ...
You can also use the ALTER PROCEDURE statement to modify the body of a stored procedure, change the permissions on a stored procedure, and rename a stored procedure.
ALTER FUNCTION
To modify a user-defined function in SQL, you can use the ALTER FUNCTION statement followed by the name of the function and the action you want to perform. For example, to modify the body of an existing function, you can use the following syntax:
ALTER FUNCTION function_name ...
You can also use the ALTER FUNCTION statement to modify the parameters of a function, change the return type of a function, and rename a function.
ALTER TRIGGER
To modify a trigger in SQL, you can use the ALTER TRIGGER statement followed by the name of the trigger and the action you want to perform. For example, to modify the body of an existing trigger, you can use the following syntax:
ALTER TRIGGER trigger_name ...
You can also use the ALTER TRIGGER statement to modify the events that the trigger fires on, rename a trigger, and disable or enable a trigger.
In conclusion, the ALTER statement in SQL is a powerful command that allows you to modify the structure of existing database objects. Whether you need to add a column to a table, modify the body of a stored procedure, or disable a trigger, the ALTER statement makes it easy to make changes to your database without having to drop and recreate objects.