Database Terms Hernandez, Chapter 3

Data/Information The values you store in the database are data. Pieces of Data in and of themselves is not particularly useful Information is processed data. It is data organized and returned in ways that give it meaning and significance

Null A null represents a missing value. It is not a 0 or an empty string “”. It is unknown. How a DBMS treats nulls can be significant. Do you count them when you are counting rows Do you include them somehow in an average by converting them to 0’s? (but this will give you a false average) Do you exclude them? (but this too may give a false average because the count could be wrong)

More on Nulls Nulls can result from error A field can be null because it is not know yet (for instance your quarter grade is null in my gradebook) A field can be null because the field does not apply to the particular record Most databases ignore nulls in aggregate functions such as sum or avg In other math functions a null +-*/ by any number is still a null

Table A two dimensional structure consisting of columns (fields) and rows (records) CustomerIDLastNameFirstNameCity 23478MartinGeorgeSeattle 34671JohnsonLarryKent 36709ThompsonChelseaSeattle

Types of Tables Data: a table that stores relevant data Lookup: a table that stores look up data, such as zip codes or state names. Helps insure data integrity and consistency Link tables: tables that link together two other tables that have a many to many relationship (Much more on this later)

Field The smallest structure in a database. A single datum. (also called an attribute or column.) In a properly designed database every field consists of one and only one value Common field problems in a poorly designed database Multipart fields (more than one distinct value) Multivalued field (multiple instances of the same type of value) Calculated fields

View A view is a virtual table composed of the fields of one or more tables A view is a way of looking at the data in the database In a good design, every database user will have an appropriate view You can use view to bring information together that is stored in separate tables You can also use views to hide information that particular users do not need to see

Keys Keys are special fields that are used to tie tables together, to relate them Primary Keys uniquely identify a row in a table (They must be unique, they cannot repeat in the table in which they are the key) Primary keys represent the table in relations A primary key can be composite (meaning it can consist of more than one field—but it is still a single key)

Foreign Keys A primary key repeated in another table to create a relationship is a foreign key. Unlike the primary key, a foreign key can repeat several times in the table. A foreign key is almost never also the primary key of a table

Relationships One to One—a single record in one table is related to one and only one record in a second table. (Each employee has one parking spot) One to Many—One record in a table can relate to many records in a second table. (One customer can make many orders) Many to Many—One student can register for many classes; each class contains many students

Integrity Table integrity (table relates only to one thing, no duplicate records) Field integrity (each field one thing, proper data type) Relational Integrity (Sound relationships, enforce things like no order without a customer) Business rules (limitations based on the business needs.)