Data Management Applications C170
What is a table? ans: A table is a persistent representation of a logical relation that is a relation whose contents can be saved for permanent use.
What are the characteristics of a relational table? ans: The characteristics of a relational table are that a ...
Data Management
Applications C170
What is a table? ans: A table is a persistent representation of a logical relation that is a relation whose
contents can be saved for permanent use.
What are the characteristics of a relational table? ans: The characteristics of a relational table are that a
table is perceived as a two-dimensional structure composed of rows and columns, each row (tuple)
represents a single entity occurrence within the set, each column represents an attribute, each column
has a distinct name, each intersection of a row and column represents a single data value, all values in a
column must be of the same data format, the order of the rows and columns don't matter to the DBMS,
and
each table must have an attribute (or combination of attributes) that uniquely identifies each row
(tuple).
What is a tuple? ans: A tuple is a row that represents a single entity occurrence within a set.
What are records? ans: A record is the same as a tuple (row).
What is a field? ans: A field is a column.
What is a key? ans: A key consists of one or more attributes that determine other attributes. For
example, a social security number identifies all of the other attributes related to that person.
What is determination? ans: Determination is the state in which knowing the value of one attribute
allows you to determine the value of another attribute. For example, if you know a key, you can find
other information.
What is functional dependence? ans: Functional dependence is when knowing the value of one or more
attributes lets you know the value of the other attributes. For example, knowing the first name and the
social security number would let you find the person's last name.
What is full functional dependence? ans: Full functional dependence is when knowing the value of one
or more attributes lets you know the value of other attributes and the value of the known attributes are
NEEDED to find the other attributes. For example, a social security number lets you find the first and last
name of someone. This is full functional dependence. If it was a social security number and a first name
this would just be functional dependence because the first name is not required to look up the last
name. All you need is the SSN.
What is a determinant? ans: A determinant is an attribute whose value determines the value of other
attributes. For example, a social security number could be the determinant for your last name.
What is a dependent? ans: A dependent is an attribute whose value is determined by another attribute.
For example, you last name could be a dependent of your social security number.
,What is a composite key? ans: A composite key is a key that is composed of one or more attribute. For
example, the key of your last name and you social security number could be used to determine your first
name. This would be a composite key because you are using your last name and your SSN in the key.
What is a superkey? ans: A superkey is a key that can uniquely identify any row in a table. Some
examples of this would be your social security number identifying a row corresponding to your
information, but a superkey isn't limited to full functional dependence. Your SSN and your last name
used as a key would still be a superkey.
What is a candidate key? ans: A candidate key is a minimal super key. This is to say it has no extra
attributes. If you only need a social security number to identify the row related to a person than the
candidate key is the SSN. A key consisting of an SSN and the last name in this case would not be a
candidate key.
What is entity integrity? ans: Entity integrity is the condition in which each row (entity instance or tuple)
in the table has it's own unique identity. This requires that all values in the primary key be unique and
that the primary key cannot be null.
What is a null? ans: A null is the absence of a data value. A null should be avoided whenever possible. In
fact, you can set a rule that prevents an attribute from being null. A null can create an issue when a
function like average is used.
What is a foreign key? ans: A foreign key is the primary key of one table that has been placed into
another table to create a common attribute. For instance, a student id number could be the primary key
for information on a student but a foreign key for a table containing a class roster.
What is referential integrity? ans: Referential integrity is the condition in which every reference to an
entity instance by another entity is valid. In other words, a foreign key needs to be either null or a valid
primary key in another table.
What is a primary key? ans: A primary key is a candidate key selected to uniquely identify all other
attributes in a given row and it may not contain null entries. Examples of this include student ID's and
social security numbers.
What is a secondary key? ans: A secondary key is a key that is used strictly for data retrieval purposes. A
secondary key does not always return a unique outcome. For instance, if a person doesn't know their
student ID, you could still look them up by their first and last name in the system but that might return
several students with the same name and you'd have to look through the list to find the appropriate
one.
What is a flag? ans: A flag is used to indicate the absence of some value to avoid using a null.
What is relational algebra? ans: Relational algebra defines the theoretical way of manipulating table
contents using relational operators.
What is a relvar? ans: A relvar is the variable that holds a relation. It is composed of two parts: a relvar
heading and a relvar body.
, What is a relvar heading? ans: The relvar heading contains the name of the attributes.
What is a relvar body? ans: A relvar body contains the relation.
What is a relation (not in general terms where you can use table and relation interchangeably)? ans: A
relation is the data that we see in our tables not the data itself. It's a relationship between sets of
information.
What is the property of closure? ans: Closure is a property that permits the use of relational operators
on existing relations to create new relations.
What is a unary operator? ans: A unary operator in regards to SQL is an operator that works on only one
table.
What is the SELECT (sometimes referred to as RESTRICT) operator used for? ans: The SELECT (or
RESTRICT) operator is used to select certain rows (aka tuples). It yields values for all rows that satisfy a
given condition. For example a SELECT statement that looks for students with a gpa below 2.0 could be
used to produce a table with all rows that contain a gpa lower than 2.0. Formally, SELECT is denoted by a
lower case sigma symbol. This is a unary operator that is it only accepts one table as input.
What is the PROJECT operator used for? ans: The PROJECT operator is a unary operator that will return
only the attributes requested that is to say it yields columns. It does not limit the rows returned so it will
return all rows within the requested columns. Formally it is denoted by the Greek letter pi.
What is the UNION operator? ans: The UNION operator combines all rows from two tables excluding
duplicate rows. To be used in the UNION the two tables must contain the same attributes. Union is
denoted by a symbol that looks like an uppercase U.
What does it mean to be union-compatible? ans: Union-compatible means that two or more tables
contain the same number of columns of the same type.
What is an INTERSECTION operator? ans: An INTERSECTION operator is an operator that yields only rows
that are in both tables. This operator requires that the tables be union-compatible. The symbol for this
operation is similar to an upside down U.
What is the DIFFERENCE operator? ans: The DIFFERENCE operator is an operator that yields all the rows
in one table that are not found in another table. It is seen as subtraction and it denoted by the minus
sign -. It's important to note that given tables A and B the DIFFERENCE between A and B is not the same
as the DIFFERENCE between B and A. Oder matters.
What is the PRODUCT operator? ans: The PRODUCT operator is an operator the yields the Cartesian
product of two tables that is it yields all possible pairs of rows of the two tables. So, if one table has 3
rows and another table has 6 rows the PRODUCT of these two tables will result in a table containing 3 x
6 rows which equals 18 rows. It is represented by the multiplication symbol.
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