Data Definition Language (DDL)

Data Definition Language (DDL) is a subset of SQL. Its primary function is to create, modify, and delete database structures but not data. The commands in DDL are:

CREATE: This command is used to create the database or its objects (like table, index, function, views, store procedure, and triggers).
```
CREATE TABLE table_name (
column1 data_type(size),
column2 data_type(size),
...
);
```
DROP: This command is used to delete an existing database or table.
```
DROP TABLE table_name;
```

ALTER: This is used to alter the structure of the database. It is used to add, delete/drop or modify columns in an existing table.

ALTER TABLE table_name ADD column_name datatype;
ALTER TABLE table_name DROP COLUMN column_name;
ALTER TABLE table_name MODIFY COLUMN column_name datatype(size);

TRUNCATE: This is used to remove all records from a table, including all spaces allocated for the records which are removed.
```
TRUNCATE TABLE table_name;
```
RENAME: This is used to rename an object in the database.
```
RENAME TABLE old_table_name TO new_table_name;
```

Remember: In DDL operations, COMMIT and ROLLBACK statement cannot be performed because the MySQL engine automatically commits the changes.

Remember to replace table_name, column_name, datatype(size), old_table_name, and new_table_name in the examples above with your actual table names, column names, data types and sizes, and the old or new table names you want to specify.