The UNIQUE constraint differs from the PRIMARY KEY constraint in what way?

Question

The UNIQUE constraint differs from the PRIMARY KEY constraint in what way?​

BlackTom AI · Accepted Answer

When evaluating how the UNIQUE constraint differs from PRIMARY KEY, we must inspect each statement carefully.
Option A: 'The UNIQUE constraint does not allow NULL values.' This is incorrect as a general rule; UNIQUE allows NULLs in many SQL dialects, meaning a column with a UNIQUE constraint can contain NULL values (often multiple NULLs). The primary key, by contrast, disallows NULLs.
Option B: 'The UNIQUE constraint can be created at either the column level or the table level.' This is true in many database systems and is a feature of UNIQUE constraints. It is a legitimate characteristic but does not capture the fundamental operational difference between UNIQUE and PRIMARY KEY with respect to NULL handling, which is the core distinction being asked.
Option C: 'The UNIQUE constraint allows NULL values.' This is the key differentiator highlighted here: unlike PRIMARY KEY, which must be NOT NULL, UNIQUE allows NULLs in most implementations. This reflects the primary contrast in how these constraints manage missing data.
Option D: 'The UNIQUE constraint ensures that a specific condition is true before a data value is added to a table.' This describes a CHECK constraint, not UNIQUE. CHECK enforces a boolean condition for insert/update, whereas UNIQUE enforces distinctness of values in a column or set of columns.
In summary, while B is a true statement about how UNIQUE can be defined, the question asks for the difference from PRIMARY KEY, and the defining difference is that UNIQUE allows NULL values (Option C). The other options either describe other constraints or misstate the nature of UNIQUE.

The UNIQUE constraint differs from the PRIMARY KEY constraint in what way?单项选择题

类似问题

Which of the following statements about the FOREIGN KEY constraint is incorrect?

Which of the following statements about creating constraints is incorrect?

Which 𝐴 𝐿 𝑇 𝐸 𝑅 𝑇 𝐴 𝐵 𝐿 𝐸 statement would be used to remove the 𝐹 𝑂 𝑅 𝐸 𝐼 𝐺 𝑁 𝐾 𝐸 𝑌 constraint named 𝐹 𝐾 𝐸 𝑚 𝑝 𝐷 𝑒 𝑝 𝑡 from the 𝐸 𝑚 𝑝 𝑙 𝑜 𝑦 𝑒 𝑒 𝑠 table?

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Which of the following statements about the 𝐶 𝐻 𝐸 𝐶 𝐾 constraint is true?

If a constraint is violated during data entry:

Which of the following statements about creating constraints is incorrect?

Which of the following SQL statements is invalid and will result in an error message? Assume that the BOOKS table currently has no constraints and contains no data. For your information, BOOKS and PUBLISHER tables have a relationship shown below:

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The UNIQUE constraint differs from the PRIMARY KEY constraint in what way?​单项选择题

类似问题

Which of the following statements about the FOREIGN KEY constraint is incorrect?​

​Which of the following statements about creating constraints is incorrect?​

Which 𝐴 𝐿 𝑇 𝐸 𝑅 𝑇 𝐴 𝐵 𝐿 𝐸 statement would be used to remove the 𝐹 𝑂 𝑅 𝐸 𝐼 𝐺 𝑁 𝐾 𝐸 𝑌 constraint named 𝐹 𝐾 𝐸 𝑚 𝑝 𝐷 𝑒 𝑝 𝑡 from the 𝐸 𝑚 𝑝 𝑙 𝑜 𝑦 𝑒 𝑒 𝑠 table?

You have a 𝑆 𝑡 𝑢 𝑑 𝑒 𝑛 𝑡 𝑠 table with an 𝐴 ≥ column. You need to enforce a rule that the age must be between 18 and 30, inclusive. Which constraint should you use and how?

Which of the following statements about the 𝐶 𝐻 𝐸 𝐶 𝐾 constraint is true?

If a constraint is violated during data entry:

Which of the following statements about creating constraints is incorrect?

Which of the following SQL statements is invalid and will result in an error message? Assume that the BOOKS table currently has no constraints and contains no data. For your information, BOOKS and PUBLISHER tables have a relationship shown below:

更多留学生实用工具

希望你的学习变得更简单

The UNIQUE constraint differs from the PRIMARY KEY constraint in what way?单项选择题

Which of the following statements about the FOREIGN KEY constraint is incorrect?

Which of the following statements about creating constraints is incorrect?