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In SQL, relations can contain null values, and comparisons with null values are treated as unknown. Suppose all comparisons with a null value are treated as false. Which of the following pairs is not equivalent?
- a)x = 5 AND not(not(x = 5))
- b)x = 5 AND x> 4 and x < 6, where x is an integer
- c)x < 5 AND not (x = 5)
- d)None of the above
Correct answer is option 'C'. Can you explain this answer?
Verified Answer
In SQL, relations can contain null values, and comparisons with null v...
For all values smaller than 5, x < 5 will always be true but x = 5 will be false.
Most Upvoted Answer
In SQL, relations can contain null values, and comparisons with null v...
Explanation:
Null values in SQL relations are treated differently from other values. Comparisons with null values are treated as unknown, and the result is neither true nor false. However, if we assume that all comparisons with null values are treated as false, then certain pairs of expressions that are logically equivalent may not be equivalent under this assumption.
a) x = 5 AND not(not(x = 5))
This expression is equivalent to x = 5, because the double negation cancels out. If x is 5, then both sides are true, and if x is not 5, then both sides are false. Therefore, this expression is equivalent to x = 5 regardless of how null values are treated.
b) x > 4 and x < 6,="" where="" x="" is="" an="" />
This expression is true if x is between 4 and 6, exclusive. If x is less than or equal to 4 or greater than or equal to 6, then it is false. If x is null, then the result is unknown, because we do not know whether x is between 4 and 6 or not.
c) x >= 5 AND not(x = 5)
This expression is not equivalent to x >= 5, because if x is null, then the second part of the expression (not(x = 5)) is also null, and the whole expression is unknown. On the other hand, x >= 5 is true if x is greater than or equal to 5, including the case where x is null.
d) None of the above
The correct answer is c), because it is not equivalent to x >= 5, which is the expression that is logically equivalent to x >= 5 AND not(x = 5) under the assumption that all comparisons with null values are treated as false.
Null values in SQL relations are treated differently from other values. Comparisons with null values are treated as unknown, and the result is neither true nor false. However, if we assume that all comparisons with null values are treated as false, then certain pairs of expressions that are logically equivalent may not be equivalent under this assumption.
a) x = 5 AND not(not(x = 5))
This expression is equivalent to x = 5, because the double negation cancels out. If x is 5, then both sides are true, and if x is not 5, then both sides are false. Therefore, this expression is equivalent to x = 5 regardless of how null values are treated.
b) x > 4 and x < 6,="" where="" x="" is="" an="" />
This expression is true if x is between 4 and 6, exclusive. If x is less than or equal to 4 or greater than or equal to 6, then it is false. If x is null, then the result is unknown, because we do not know whether x is between 4 and 6 or not.
c) x >= 5 AND not(x = 5)
This expression is not equivalent to x >= 5, because if x is null, then the second part of the expression (not(x = 5)) is also null, and the whole expression is unknown. On the other hand, x >= 5 is true if x is greater than or equal to 5, including the case where x is null.
d) None of the above
The correct answer is c), because it is not equivalent to x >= 5, which is the expression that is logically equivalent to x >= 5 AND not(x = 5) under the assumption that all comparisons with null values are treated as false.
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In SQL, relations can contain null values, and comparisons with null values are treated as unknown. Suppose all comparisons with a null value are treated as false. Which of the following pairs is not equivalent?a)x = 5 AND not(not(x = 5))b)x = 5 AND x> 4 and x < 6, where x is an integerc)x < 5 AND not (x = 5)d)None of the aboveCorrect answer is option 'C'. Can you explain this answer? for Computer Science Engineering (CSE) 2025 is part of Computer Science Engineering (CSE) preparation. The Question and answers have been prepared according to the Computer Science Engineering (CSE) exam syllabus. Information about In SQL, relations can contain null values, and comparisons with null values are treated as unknown. Suppose all comparisons with a null value are treated as false. Which of the following pairs is not equivalent?a)x = 5 AND not(not(x = 5))b)x = 5 AND x> 4 and x < 6, where x is an integerc)x < 5 AND not (x = 5)d)None of the aboveCorrect answer is option 'C'. Can you explain this answer? covers all topics & solutions for Computer Science Engineering (CSE) 2025 Exam. Find important definitions, questions, meanings, examples, exercises and tests below for In SQL, relations can contain null values, and comparisons with null values are treated as unknown. Suppose all comparisons with a null value are treated as false. Which of the following pairs is not equivalent?a)x = 5 AND not(not(x = 5))b)x = 5 AND x> 4 and x < 6, where x is an integerc)x < 5 AND not (x = 5)d)None of the aboveCorrect answer is option 'C'. Can you explain this answer?.
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