Definition of DDL.Examples of DDL commands (CREATE, ALTER, DROP).Use cases and scenarios where DDL is employed.

DDL, or Data Definition Language, is a type of SQL (Structured Query Language) used to define and manage all the objects in an SQL database. It provides commands for defining, altering, and deleting database structures but not data.

Examples of DDL commands include:

1. CREATE: This command is used to create the database or its objects like table, index, function, views, triggers, etc. For example, "CREATE TABLE table_name (column1 datatype, column2 datatype, column 3 datatype);"

2. ALTER: This command is used to alter the structure of the database. For example, "ALTER TABLE table_name ADD column_name datatype;" This command adds a new column to the table.

3. DROP: This command is used to drop an existing database or table. For example, "DROP TABLE table_name;" This command will delete the entire table from the database.

Use cases and scenarios where DDL is employed:

1. Database Creation: When a new database is to be created, DDL commands are used. The CREATE command is used to create a new database, define its structure, create tables, define primary and foreign keys, etc.

2. Database Modification: If there is a need to modify the structure of an existing database, like adding a new column to a table or changing the data type of a column, ALTER command is used.

3. Database Deletion: When there is a need to delete an entire database or some tables from the database, DROP command is used.

4. Index Creation: Indexes are used to retrieve data from the database more quickly than otherwise. The CREATE INDEX command is used to create these indexes.

5. View Creation: Views are virtual tables based on the result-set of an SQL statement. The CREATE VIEW command is used to create views.