Table Joins - Quiz Explanation

The answers you selected are indicated below, along with text that explains the correct answers.

1.	For a table join to produce meaningful information, the tables must be linked by what kind of fields? Please select the best answer.
	A.	Fields of the same name
	B.	Fields with the same data domain
	C.	Fields with equivalent data
	D.	Fields that are both primary-key fields in their respective tables
	The correct answer is C. For a table join to produce meaningful information, the joined tables must be linked by fields that contain equivalent data. A is incorrect because, while equivalent fields will often have the same name, it is the data they contain that must be the same, not their names. B is incorrect because having fields with the same data domain is necessary but not sufficient to produce a meaningful join. A customer's ID and the quantity of items ordered might both be stored as INTEGERS, but joining tables where a CustID equaled an order quantity would be meaningless. D is incorrect because it is not necessary that linked fields both are primary-key fields in their respective tables. It is possible to produce meaningful information by joining two tables where only one of the linked fields is a primary-key field for its tables.

2.	When you link two tables with an outer join, what character does the RDBMS use to fill in empty columns when a record has no matching record in the other table? Please select the best answer.
	A.	Blank
	B.	Null
	C.	Zero
	D.	Question mark
	The correct answer is B. When you link two tables with an outer join, the RDBMS will fill empty columns with null values when a record in one table has no matching record in the other. This indicates the value may exist but is unknown. A is incorrect because a blank indicates there is definitely not a value for the column, which may or may not be true. C is incorrect because a zero could only be used for fields with a numerical domain, and would indicate that the value in that field is 0, not that the record has no match. D is incorrect because a question mark has no special meaning within a table column and is treated as a text string.

3.	A view creates what type of tables for users to manipulate? Please select the best answer.
	A.	A joined table
	B.	A base table
	C.	A virtual table
	D.	A linked table
	The correct answer is C. A view creates a virtual table for users to manipulate. A is incorrect because, while a view can be based on joined tables, they are not a prerequisite for creating a view. B is incorrect because the purpose of creating a view is to remove the need for users to work with the base tables. D is incorrect because, while a view can be based on linked tables, they are not a prerequisite for a view.

4.	A view is stored as what type of database object? Please select the best answer.
	A.	A base table
	B.	An SQL query
	C.	A virtual table
	D.	A relation
	The correct answer is B. A view is stored as an SQL query. A is incorrect because the purpose of creating a view is to remove the need for users to work with the base tables. C is incorrect because a view creates a virtual table but is not itself a virtual table. D is incorrect because a relation is a construct that defines the structure of a table, not how that table's contents are presented to users.