In SQL, data definition language (DDL) is use for
creating and modifying database and its objects like
tables, indexes, and users. In Data Definition Language
include ( Create, Alter, Drop ) Commands.

Creating Database:
create database safcode

Creating Table:
Syntax:
create table table_name (
    column1 datatype, 
    column2 datatype,
    column3 datatype,
   ....
)
//like int, varchar, date etc

Example:
create table courses (
    courseid int,
    coursename varchar(50),
    duration varchar(50),
    fee int
)

Alter Table:
If you want to add, remove or modify your columns in
an existing table so you can use alter command.

add addmission_date columns in a existing table.

Example:
alter table courses
add addmision_date varchar(50)

Delete Column:
alter table courses
drop column addmision_date

Modify Column:
alter table courses
alter column addmision_date date

DROP Command:
drop table table_name/database_name

Example:
drop table courses

The below SQL Server tutorial will help you to learn
this DDL all Commands