The explanation to question 3 is that UNIQUE allows us to put just one NULL value. Then, the answer to question 4 should be that we cannot add a record with another NULL value for ID, because the ID must be unique! Why the correct answer to question 4 is another one, instead? Thank you

Hi Alessandro! Thanks for reaching out. Could you please explain which exercises do you refer to? Thank you. Looking forward to your answer. Best, Tsvetelin

Good morning. I refer to "Practice Exam: 1" at the end of chapter 9. The explanation to question 3 says "the former allows us to put only one NULL value", and it refers to UNIQUE keys. Is the explanation to question 3 wrong or is the answer to question 4 wrong? I guess it's the former. Thank you