Which does not form the basis of a relational model?
(a.) Which of the below does not form the basis of a relational model?
- Every column in a table represents an attribute of the entity
- All rows in a table combine together to represent a single instance of the entity in the table
- Data is represented by a series of two-dimensional tables
- Each column and row intersection should have an atomic value
Answer
b is the correct answer.
The relational table is nothing but an entity and all the properties of that entity are the columns of the table, which are also known as attributes.
Hence, option-a forms a basis of a relational model.
Option-b states wrong about a relational table.
Generally, we represent an instance of a relational table as a row of a table (tuple). And, all the combined rows are not all instances.
Hence this is wrong.
The relational table (entity), will have attributes (columns) and several instances (rows), hence we represent it as two-dimensional tables.
This is correct.
A cell is the intersection of the row and column of a table. Each cell in the relational table must be atomic.
This is true too.
Hence, the correct option which does not form a basis of the relational model is option-b.
(b.) Which of the below is a mandatory clause in an old SQL Join statement?
- WHERE
- USING
- ON
- ORDER BY
Answer
c is the correct answer.
The clause “ON” is mandatory in SQL join statement.
Example:
Select * from Product p
Inner join Supplier s
On p.productID = s.productID;
In the above example, two entities with the name Product and Supplier are present and those two tables are related via a foreign key productid in the supplier table. Hence, the “ON” clause is necessary to match the records that link both the tables.
Hence, option – c is correct.
(c.) Which amongst the below is not a good database design rule?
- All calculated values that are directly derived from other fields should be part of the table
- Must avoid redundant data
- Every table must have a unique identifier or primary key
- Every cell in the database must have atomic values
Answer
a is the correct answer.
The design rule “All calculated values that are directly derived from other fields should be part of the table” is not a good database design rule.
To optimize the storage, one must avoid redundant data. In fact, we have designed the DBMS concept to majorly manage storage by avoiding redundancies.
Also, to relate several tables by means of a foreign key, every table must have a primary key which would be the unique identifier of that table.
Also, every attribute of the table should have only atomic values in their domain.
But, having calculated values that are derived from other fields, would in fact increase redundancies which spoil the storage optimization.
Hence, option-a is correct.
(d.) Which of the below define(s) data redundancy in an RDBM
- Same data is stored in more than one location
- Same data is stored multiple times in one location
- Both of the above
- None of the above
Answer
a is the correct answer.
Generally, the term “data redundancy” means the occurrence of the same piece of data in multiple locations.
Here, option-a has described that data redundancy in RDBMS is “storing the same data in more than one location”.
This is the same as our description, so correct.
In option-b, it says, data redundancy means storing multiple times the same data in one location. This does not specify the data redundancy.
Hence, this is wrong.
So, the correct option is option-a.