SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL;

SELECT *
FROM Employee
WHERE email IS 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 in creating a table
CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255) NOT NULL, Age int
);

CREATE TABLE Colleges ( college_id INT NOT NULL, college_code VARCHAR(20), college_name VARCHAR(50)
);

select c_custkey, c_name, c_address from customer where c_custkey in (select o_orderkey from orders where o_orderkey IS NOT NULL)

CREATE TABLE Colleges ( college_id INT NOT NULL, college_code VARCHAR(20) NOT NULL, college_name VARCHAR(50)
);

CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255) NOT NULL, Age int
);