Which of the following is a valid SQL statement to add a primary key to an existing table? MCQ with Answer and Explanation

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Which of the following is a valid SQL statement to add a primary key to an existing table?

A. ADD PRIMARY KEY ON table_name (column_name);

B. ALTER TABLE table_name ADD CONSTRAINT pk_name PRIMARY KEY (column_name);

C. ALTER TABLE table_name ADD PRIMARY KEY (column_name);

D. Both A and C

Answer: Option D

Solution (By JKExamLibrary)

Both syntaxes are valid: ADD PRIMARY KEY (column) and ADD CONSTRAINT ... PRIMARY KEY (column).

This question belongs to: Computer Database Management System (DBMS)

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Question #1

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The range of all permissible and valid values that an attribute can hold is formally defined as its what?

A. Domain

B. Relation

C. Tuple

D. Schema

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Correct Answer: Option A

Explanation:
A Domain is the pool of legal, atomic values from which an attribute can take its value.

This question belongs to: Computer Database Management System (DBMS)

Question #2

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Which type of join returns all rows from the left table and matching rows from the right table?

A. INNER JOIN

B. LEFT JOIN

C. RIGHT JOIN

D. FULL JOIN

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Correct Answer: Option B

Explanation:
LEFT JOIN (or LEFT OUTER JOIN) returns all rows from the left table, with matching rows from the right table; NULLs appear when no match.

This question belongs to: Computer Database Management System (DBMS)

Question #3

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Which of the following is a type of database backup?

A. Full backup

B. Incremental backup

C. Differential backup

D. All of the above

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Correct Answer: Option D

Explanation:
Full, incremental, and differential backups are common backup strategies.

This question belongs to: Computer Database Management System (DBMS)

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