Study Guide Mis 691 Chapter 2

Study Guide MIS 691 Midterm Chapter 2 2. 2 Entity un run intoed theme or topic something of importance to a giver that require to be represented in a entropybase. In the entity- congenership model, entities atomic number 18 restrict to things that tail end be represented by a single prorogue. An mannequin of this is in the EMPLOYEE side flavor data about the entity (the employee) is in the mesa tracks and data about the entitys attributes are in the rows. 2. List the characteristics a table must have to be a relation * Each row of the table holds data that pertain to some entity or a batch of some entity * Each towboat contains data of the attributes of the entity * The cells must hold a single repute no repeating elements * All of the entries in either column must be the same kind * Each column must have a unique name and the column narrate is unimportant * The order of the rows in unimportant * No both rows in the table may hold identical data set 2. 0 Define the term unique bring up and give an specimen A account that defines a unique row. An example of a unique pick out in the EMPLOYEE table is EmployeeNumber the query of all EmployeeNumber (223 for example) will only prepare a single row and data for single employee. 2. 11 Non-unique disclose A pick up that potentially identifies to a greater extent than one row. In the EMPLOYEE table from our book, division is a non-unique key because it identifies several rows. 2. 12 Give an example of a relation with a unique complex key A composite key contains two or more attributes.An example of a unique composite key in the EMPLOYEE table is if we combined Last arouse, FirstName, and Department as a key these lead attributes combined would insure that we would delineate only one row. 2. 13 Explain the difference between a primary key and a candidate key Candidate keys are keys that unequivocally identify apiece row in a relation. They can be single or composite. The primary key is the candidate key that is chosen as the key that the DBMS will use to uniquely identify each row. 2. 5 What is a surrogate key and chthonian what circumstances would you use one? A surrogate key is a column with a unique, DBMS-assigned identifier that has been added to a table to be the primary key. You would use a surrogate key when the primary key in the table is not ideal. You add a surrogate because it is short, numeric, and will never convince it is an ideal primary key. 2. 20 Define the term referential rightfulness restraint and give an example A relationship constraint on foreign key values.A referential integrity constraint specifies that the values of a foreign key must be a proper subset of the values of the primary key to which it refers. In the EMPLOYEE table the value of Department should match a value of DepartmentName in the DEPARTMENT table. 2. 21 Explain the three possible interpretations of a null value 1. It could mean that no value is appropriate 2. It might mea n that the value is known to be unclouded (intentionally left blank) 3. It may mean that the value is unknown 2. 4 Name the functional dependency and identify the determinants of Area = Length x largeness Area is functionally dependent upon Length and Width or that the composite of Length and Width are determinants of Area 2. 31 Describe the nature and purpose of the normalization process Normalization is breaking up tables with more than one theme into sets of tables that have one theme each. You have to do this so that there isnt repeat information in a table and so there are no modification or deletion issues.Normalization Normalization is the process of breaking a table with one or more theme into a set of tables such that each only has one theme. You need to create a well-formed relation 1. Every determinant must be a candidate key 2. Any relation that is not well-formed should be broken into two or more relations that are well-formed The Normalization dish 1. Indentify all of the candidate keys 2. Indentify all of the functional dependencies 3. Examine the dereminants of the functional dependencies.If any determinant is not a candidate key, the relation is not well-formed. In this case a. Place the columns of the functional dependency in a saucily relation of their own b. Make the determinant of the functional dependacy the primary key of the new relation c. Leave a copy of the determinat as a foreign key in the original relation d. Create a referential integrity constraint between the orginal and the new relation. 4. Repeat step 3 until every key is a candidate key