Question
In a relational database, which key uniquely identifies
tuples within a table and can never contain NULL values?Solution
A Primary Key uniquely identifies each record (or tuple) in a table and cannot contain NULL values. It enforces entity integrity, ensuring that every row in a table has a unique identifier. For example, in a table of employees, an employee ID serves as a primary key, ensuring no duplicate or NULL entries exist. Why Other Options Are Incorrect:
- Foreign Key: Links one table to another and can contain NULL values in some cases; its purpose is to enforce referential integrity, not uniqueness within its table.
- Candidate Key: A potential key that can uniquely identify tuples but is not chosen as the primary key.
- Alternate Key: A candidate key not selected as the primary key but still capable of uniquely identifying records.
- Composite Key: A primary key consisting of multiple columns but is a type of primary key, not a separate concept.
Statements: A ≥ B ≥ Y = Z = M ≥ N ≤ E ≤ F = J
Conclusions:
I. F > Z
II. J ≤ Y
...Statements: F > V > W ≥ L > G; F ≤ O = M < I
Conclusions: I. M > LÂ Â Â II. V < I
In the question, assuming the given statements to be true, find which of the conclusion (s) among given three conclusions is /are definitely true and t...
Statement: A ≥ B ≥ C = D > E, F > G = H ≤ CÂ
Conclusion: I. C ≥ F                         II. F > D
...In the question, assuming the given statements to be true, find which of the conclusion (s) among given three conclusions is/are definitely true and th...
Statements: T < Q ≥ L; W < Q ≥ E; E < S
Conclusions:
I. T < S
II. S > Q
III. E < L
Statements:
B > L > M ; M = W ; L < Q
Conclusions:
I. W < L
II. M < B
III. B > Q
Statements : R < O ≤ P < Q; M < L ≤ N > O; S < Q ≤ K
Conclusions:
I. M < K
II. N > R
III. O < S
Statements: P < Q = S ≥ U; V ≤ P ≥ N > I
Conclusions: I. U < V II. Q > I
...Statements: J ≤ K = L ≤ N; N ≤ O = M
Conclusions: I. M = JÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â II. J < M
...
