The syntax for filtering for NULL (i.e. the absence of a value) in WHERE blocks is slightly different than filtering for specific values.

SELECT * FROM Employees WHERE ManagerId IS NULL ;
SELECT * FROM Employees WHERE ManagerId IS NOT NULL ;

Note that because NULL is not equal to anything, not even to itself, using equality operators = NULL or <> NULL (or != NULL) will always yield the truth value of UNKNOWN which will be rejected by WHERE.

WHERE filters all rows that the condition is FALSE or UKNOWN and keeps only rows that the condition is TRUE.

When creating tables it is possible to declare a column as nullable or non-nullable.

CREATE TABLE MyTable
(
    MyCol1 INT NOT NULL, -- non-nullable
    MyCol2 INT NULL      -- nullable
) ;

By default every column (except those in primary key constraint) is nullable unless we explicitly set NOT NULL constraint.

Attempting to assign NULL to a non-nullable column will result in an error.

INSERT INTO MyTable (MyCol1, MyCol2) VALUES (1, NULL) ;  -- works fine

INSERT INTO MyTable (MyCol1, MyCol2) VALUES (NULL, 2) ;  
        -- cannot insert
        -- the value NULL into column 'MyCol1', table 'MyTable'; 
        -- column does not allow nulls. INSERT fails.

Setting a field to NULL works exactly like with any other value:

UPDATE Employees 
SET ManagerId = NULL
WHERE Id = 4

For example inserting an employee with no phone number and no manager into the Employees example table:

INSERT INTO Employees
    (Id, FName, LName, PhoneNumber, ManagerId, DepartmentId, Salary, HireDate)
VALUES
    (5, 'Jane', 'Doe', NULL, NULL, 2, 800, '2016-07-22') ;