If order_id is declared as a PRIMARY KEY, which of the following are guaranteed?

Choose all that apply.

Question

If order_id is declared as a PRIMARY KEY, which of the following are guaranteed?

Choose all that apply.

BlackTom AI · Accepted Answer

A PRIMARY KEY in SQL enforces two logical constraints: the column must be NOT NULL and it must uniquely identify rows (uniqueness). These are the relevant knowledge points.

1) Relevant concepts
- NOT NULL: the column cannot contain NULL values.
- UNIQUE: no two rows can have the same value in the primary key column.
- SQL standard vs. implementation details: the standard requires NOT NULL and uniqueness; physical implementation details (indexes, auto-increment behavior) are DBMS-specific and not part of the semantic guarantee.

2) Step-by-step reasoning
- Declaring order_id PRIMARY KEY tells the database that order_id is the table’s identifier for rows.
- The SQL standard defines a primary key as a candidate key that is designated for identifying rows; a candidate key must be non-nullable and unique.
- Therefore, the declaration causes the column to be implicitly NOT NULL (you need not explicitly write NOT NULL), and the DBMS will enforce uniqueness for that column.

3) Intermediate “steps” (conceptual)
- Before declaration: order_id could allow NULLs and duplicates.
- After declaration: the DBMS rejects INSERT/UPDATE that would produce NULL in order_id or create a duplicate value.
- Implementation note: many DBMS also create a unique index to enforce uniqueness, but that is an implementation mechanism, not the semantic guarantee.

4) Why the correct answer is correct
- The provided answer, "order_id is implicitly NOT NULL," is guaranteed by the PRIMARY KEY declaration: the SQL standard requires primary key columns to be NOT NULL, so the DBMS treats the column as if you had declared NOT NULL explicitly.

5) Why other common choices are incorrect or not guaranteed
- “Automatically auto-incremented” is not guaranteed by PRIMARY KEY; auto-increment is a separate column property.
- “An index is always created” is typical but implementation-specific; the standard does not require a particular physical index, only uniqueness.
- Any option suggesting only NOT NULL but not uniqueness would be incomplete; uniqueness is also logically enforced (but the provided answer correctly identifies the NOT NULL implication).

Conclusion: The statement "order_id is implicitly NOT NULL" is correct and matches the provided answer.

If order_id is declared as a PRIMARY KEY, which of the following are guaranteed? Choose all that apply. 单项选择题

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