SQL Not null constraint part 2

In part SQL Not null constraint 1
https://youtu.be/kYkAumb9uBI
In this video we will learn about SQL Constraints:

SQL constraints are used to specify rules for data in a table.
These rules control the data that can be stored in a column.
Creating Constraints:
Constraints can be specified when the table is created with the CREATE TABLE statement, or after the table is created with the ALTER TABLE statement.

Syntax:
CREATE TABLE table_name (
column1 datatype constraint,
column2 datatype constraint,
column3 datatype constraint,
....
);

1. NOT NULL:

By default, a column can hold NULL values.
The NOT NULL constraint enforces a column to NOT accept NULL values.
This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.

SQL NOT NULL on CREATE TABLE:

Query: Create an Employee table with columb names, Empid, Empname and country.
Empid, Empname columns will NOT accept NULL values

Ex:
CREATE TABLE Employee(
Empid int Not Null,
EmpName varchar(255) Not Null,
Country varchar(50) );
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SQL NOT NULL on ALTER TABLE:

Syntax:
Alter table tablename
Alter Column columnname datatype constraint;

Example:
Alter table Cmdtable
Alter column Sno int not null

Now when we try to insert records in a Colleges table without value for college_id, SQL will give us an error. For example,

Insert into Cmdtable(FullName, City, DOB)
values('Steve', 'Newyork', '1985-02-31')
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Drop Not null contstraint:

Syntax:
Alter table tablename
Alter Column columnname datatype

Example:
Alter table Cmdtable
Alter column Sno int