Chapter 3

Question 1

What are the characteristics of a relational table?

Answer 1

1) A table is perceived as a 2-dim structure composed of rows and columns.

2) Each table row represents a single entity occurrence within the entity set.

3) Each table column represents an attribute, and each column has a distinct name.

4) Each intersection of a row and column represents a single data value.

5) All values in a column must conform to the same data format.

6) Each column has a specific range of values knows as the attribute domain.

7) The order of the rows and columns is immaterial to the DBMS.

8) Each table must have an attribute or combination of attributes that uniquely identifies each row (each row has a unique entity, that distinguish these records from others).

Question 2

What is a primary key?

Answer 2

Is a key that will identify the whole row (record). For example, the student number (n…) is the primary key that uniquely identifies any given row. Can be a single or, two or more. You have to set it up (not automatic).

All of the values in the primary key must be unique.

No key attribute in the primary key can contain a null

Question 3

What is a foreign key?

Answer 3

The primary key of one table that has been placed into another table to create common attribute

Question 4

What is Functional dependence?

Answer 4

Value of one or more attributes determines the value of one or more other attributes

Question 5

What is Determinant?

Answer 5

Attribute whose value determines another. only the primary key is called determinate.

Question 6

What is Dependent?

Answer 6

Attribute whose value is determined by the other attribute

Question 7

What is Superkey?

Answer 7

A key that can uniquely identify any row in the table.

Question 8

What is Secondary key?

Answer 8

Key used strictly for data retrieval purposes.

Question 9

What are the ways of handling Nulls?

Answer 9

Flags: Special codes used to indicate the absence of some value (pop-up messages).

NOT NULL constraint, For example, we cannot duplicate student number (N…)

UNIQUE constraint

Question 10

What is relational algebra?

Answer 10

Theoretical way of manipulating table contents using relational operators.

Question 11

What is a Relvar?

Answer 11

Variable that holds a relation.

Question 12

What is a closure?

Answer 12

Use of relational algebra operators on existing relations produces new relations.

Question 13

What is select as a relational operator?

Answer 13

Filtering in the database. You are selecting pieces or all data of the database. For example, “Select all” or “Select only price less than $2.00 yields”.

Question 14

What is project as a relational operator?

Answer 14

A projecting (focusing) what you are looking for. Will choose one attribute (not the whole record) –> creating a new table. Väljer en kolumn och skapar en ny tabell

Question 15

What is union as a relational operator?

Answer 15

Unionizing (adding) tables. Do not have to repeat if they are already in the table (you do not want duplicates).

Question 16

What is Intersect as a relational operator?

Answer 16

What is in common between two tables; intersect them.

Question 17

What is difference as a relational operator?

Answer 17

Is the difference between table 1 to table 2 (not the vice versa!), that is subtract table right from the table 1. Table 1 will have a precedence (företräde) of table 2.

Yields all rows in one table that are not found in theother table

Question 18

What is Product as a relational operator?

Answer 18

Multiply each row(record) in table 1 with each row in table 2.

Question 19

What is a join?

Answer 19

Allows information to be intelligently combined from two or more tables

Question 20

What is a Natural join (product)?

Answer 20

Links tables by selecting only the rows with common values in their common attributes

Question 21

What is an Equijoin?

Answer 21

Links tables on the basis of an equality condition that compares specified columns of each table

Question 22

What is a Theta join?

Answer 22

Extension of natural join, denoted by adding a theta subscript after the JOIN symbol

Question 23

What is an Outer join?

Answer 23

Matched pairs are retained and unmatched values in the other table are left null

Question 24

What is a Left outer join?

Answer 24

Yields all of the rows in the first table,including those that do not have a matching value in the second table

Question 25

What is a Right outer join?

Answer 25

Yields all of the rows in the second table, including those that do not have matching values in the first table

Question 26

What is Divide?

Answer 26

Accuracies. Uses one 2-column table as the dividend and one single-column table as the divisor. Output is a single column that contains all values from the second column of the dividend that are associated with every row in the divisor.