# RDBMS Questions and Answers – Estimating Statistics of Expression Results

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This set of RDBMS Multiple Choice Questions & Answers (MCQs) focuses on “Estimating Statistics of Expression Results”.

1. Which of the following information does the database system catalog store?
a) Number of tuples
b) Number of blocks
c) Size of a tuple of a relation
d) All of the mentioned

Explanation: The database system catalog stores the number of tuples, number of blocks, size of a tuple in a relation and the blocking factor in it as statistical data.

2. Most databases store the distribution of values for each attribute as a __________
a) Histogram
b) Pie chart
c) Line graph
d) None of the mentioned

Explanation: Most of the databases store the distribution of values for each attribute as a histogram.in which the values for the attributes are divided into various ranges.

3. What is the function of the equi-width histogram?
a) Adjusts boundaries of the ranges such that each range has the same number of values
b) Divides range of values into equal sized ranges
c) Divides the range of values into ideally sized ranges
d) Does not divide the range of values.

Explanation: The equi width histogram divides the range of values into equal sized ranges.
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4. What kind of a sample must be used for statistical analysis?
a) A random sample
b) A sample having excessive representation of a relation
c) A sample having suppressive representation of a relation
d) None of the mentioned

Explanation: A random sample must be taken into consideration for statistical analysis because it gives a fair result on the data analysis.

5. The ________ SQL command generates statistics on a particular relation
a) Statistic
b) Analyze
c) Modify
d) Runstats

Explanation: The analyze command is used to generate statistics on a particular set of relations. The runstats command is also valid but not in SQL. It is used in IBM DB2.

6. The union of all records satisfying the individual simple conditions Oi is called as ________
a) Conjunctive selection
b) Disjunctive selection
c) Negation
d) None of the mentioned

Explanation: The union of all records satisfying the individual simple conditions Oi is called as disjunctive selection. It is a form of a complex selection.

7. The intersection of all records satisfying the individual simple conditions Oi is called as ________
a) Conjunctive selection
b) Disjunctive selection
c) Negation
d) None of the mentioned

Explanation: The intersection of all records satisfying the individual simple conditions Oi is called as conjunctive selection. It is a form of a complex selection.

8. State true or false: Estimation of the size of the result of a join is not possible
a) True
b) False

Explanation: Estimation of the result of a join is possible and can be done using the Cartesian product and intersections of the two relations. It is more complicated than estimating size of a selection.

9. Size estimation can be done for which of the following processes?
a) Projection
b) Aggregation
c) Set operation
d) All of the mentioned

Explanation: Size estimation can be done for projection, aggregation, set operations, join operations and also the selection operation.

10. The size of a _____ is simply V(A,r) where r is the relation and A is a distinct value
a) Projection
b) Outer join
c) Aggregation
d) Inner join

Explanation: The size of an aggregation is simply V(A,r) where r is the relation and A is a distinct value since there is one tuple in the aggregation for every distinct value in A.

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