ER and Relational Models

Question 41

If D₁, D₂...D_n are domains in a relational model, then the relation is a table, which is a subset of

• D₁⊕D₂⊕...⊕D_n
• D₁xD₂x...xD_n
• D₁∪D₂∪...∪D_n
• D₁∩D₂∩...∩D_n

Question 42

Relational database schema normalization is NOT for:

• reducing the number of joins required to satisfy a query.
• eliminating uncontrolled redundancy of data stored in the database.
• eliminating number of anomalies that could otherwise occur with inserts and deletes.
• ensuring that functional dependencies are enforced.

Question 43

Consider the following statements regarding relational database model: (a) NULL values can be used to opt a tuple out of enforcement of a foreign key. (b) Suppose that table T has only one candidate key. If Q is in 3NF, then it is also in BCNF. (c) The difference between the project operator (Π) in relational algebra and the SELECT keyword in SQL is that if the resulting table/set has more than one occurrences of the same tuple, then Π will return only one of them, while SQL SELECT will return all. One can determine that:

• (a) and (b) are true.
• (a) and (c) are true.
• (b) and (c) are true.
• (a), (b) and (c) are true.

Question 44

Consider the following Entity-Relationship (E-R) diagram and three possible relationship sets (I, II and III) for this E-R diagram: I II II If different symbols stand for different values (e.g., t₁ is definitely not equal to t₂), then which of the above could not be the relationship set for the E-R diagram ?

• I only
• I and II only
• II only
• I, II and III

Question 45

An aggregation association is drawn using which symbol?

• A line which loops back on to the same table
• A small open diamond at the end of a line connecting two tables
• A small closed diamond at the end of a line connecting two tables
• A small closed triangle at the end of a line connecting two tables

Question 46

Every time the attribute A appears, it is matched with the same value of attribute B but not the same value of attribute C. Which of the following is true?

• A-> (B,C)
• A ->B, A->>C
• A->B, C->>A
• A->>B, B->C

Question 47

Suppose database table T₁(P, R) currently has tuples {(10, 5), (15, 8), (25, 6)} and table T₂ (A, C) currently has {(10, 6), (25, 3), (10, 5)}. Consider the following three relational algebra queries RA₁, RA₂ and RA₃: 
RA₁ : T₁ ⨝ _{T1.P = T2.A} T₂ where ⨝is natural join symbol 
RA₂ : T₁ ⟕ _{T1.P = T2.A} T₂ where ⟕ is left outer join symbol 
RA₃ : T₁ ⨝ _{T1.P = T2.A and T1.R = T2.C}T₂
The number of tuples in the resulting table of RA₁, RA₂ and RA₃ are given by:

• 2, 4, 2 respectively

• 2, 3, 2 respectively

• 3, 3, 1 respectively

• 3, 4, 1 respectively

Question 48

Which of the following statement(s) is/are FALSE in the context of Relational DBMS ? I. Views in a database system are important because they help with access control by allowing users to see only a particular subset of the data in the database. II. E-R diagrams are useful to logically model concepts. III. An update anomaly is when it is not possible to store information unless some other, unrelated information is stored as well. IV. SQL is a procedural language.

• I and IV only
• III and IV only
• I, II and III only
• II, III and IV only

Question 49

Consider the following two commands C1 and C2 on the relation R from an SQL database: C1 : drop table R; C2 : delete from R; Which of the following statements is TRUE ? I. Both C1 and C2 delete the schema for R. II. C2 retains relation R, but deletes all tuples in R. III. C1 deletes not only all tuples of R, but also the schema for R.

• I only
• I and II only
• II and III only
• I, II and III

Question 50

Consider the following database table having A, B, C and D as its four attributes and four possible candidate keys (I, II, III and IV) for this table : I : {B} II : {B, C} III : {A, D} IV : {C, D} If different symbols stand for different values in the table (e.g., d₁ is definitely not equal to d₂), then which of the above could not be the candidate key for the database table ?

• I and III only
• III and IV only
• II only
• I only