In SQL, the ALTER TABLE statement is used to modify the structure of an existing table. It allows you to add, modify, or drop columns, constraints, and other properties of a table. The syntax for the ALTER TABLE statement varies slightly depending on the specific database management system (DBMS) you are using. Here's a general example:

sql
ALTER TABLE table_name
    action1,
    action2,
    ...

- `table_name`: The name of the table you want to modify.
- `action1`, `action2`, etc.: The specific actions you want to perform, such as adding or modifying columns, constraints, or other table properties.

Here are some common actions that can be performed using the ALTER TABLE statement:

1. Add a column:

sql
ALTER TABLE table_name
    ADD column_name data_type;

2. Modify a column:

sql
ALTER TABLE table_name
    ALTER COLUMN column_name new_data_type;

3. Drop a column:

sql
ALTER TABLE table_name
    DROP COLUMN column_name;

4. Add a constraint:

sql
ALTER TABLE table_name
    ADD CONSTRAINT constraint_name constraint_definition;

5. Drop a constraint:

sql
ALTER TABLE table_name
    DROP CONSTRAINT constraint_name;

These are just a few examples of the actions that can be performed using the ALTER TABLE statement. The specific options and syntax for each action may vary depending on your DBMS.

It's important to note that altering a table can have implications on the existing data and applications that rely on the table structure. Care should be taken when making modifications to ensure data integrity and consistency.

Consult the documentation or specific syntax guidelines for your chosen DBMS for detailed information on using the ALTER TABLE statement to modify tables.