Relational Databases

CIS-182

What is a database?

• Database: Collection of related data and the tools to manage and use that data

• Collection refers to a group of like things– Students at SPSCC represent a collection

• What belongs in the group is determined by a purpose, task or need: – What will the data be used for?– Provides the ability to determine what specific

data is needed to complete a task, satisfy the stated purpose

Manage & Use Data

• Add new data• Edit existing data• Remove data• Find data– Filtering: limit by characteristics– Sorting: order by value

Database Tools

• How data is stored doesn’t matter– May be a list– May be post-it notes

• Tools may be simple or complex– Piece of paper and pencil– Spreadsheet

• A Grocery List is a database– Using pen/paper, can add items to buy, change

items to buy, remove items to buy– Use a different list for different days or stores

What is a relational database?

• Incorporates basic definition of a database• Data organized in a set of tables, where the data

and relationships between data are modeled based on the real world– Table: Group of records about one kind of

thing (entity)– Record: Entry for one entity (row)– Field: Single value describing characteristic of one

entity (column)

• Reduces data entry, size of files, number of errors

• Helps to ensure the accuracy and validity of data

Relationships – 1:1

• One to one: for each record in one table there is a single corresponding record in a second table

• Similar to splitting a table in two: – If have a persons table (name, address) could

have a students table (school ID, major); each person can only be a single student

Relationships – 1:M

• One to many: for each record in one table, there can be one or more related records in a second table

• In simplest form, represents ownership– One student completes many assignments

• Each assignment “belongs” to a single student, and only that student

Relationships – M:N

• Many to many: for each record in both tables, there can be many matching records in the other table

• Most common kind of relationship– One student takes many classes, each class has

many students

• Requires a third table to create relationship– Third table “joins” entries in original table

• An Enrollments table would identify which student is in which class

– Join table has at least two foreign keys

Primary Keys

• Keys provide a means to find specific rows • Primary key defines unique, required value

in a table– Provides a way to get one row in the table– May be one or more columns

• Column(s) in primary key must have a value• Value(s) must be different for each row• Table can have only one primary key

• Student ID represents a value that is different (unique) for each student in the Students table

Foreign Keys

• Foreign key is a value in one table that refers to a unique value in a different table– “Foreign” means outside

• A student ID in enrollments refers to an entry in the Students table

– Usually refers to primary key, but can use any unique index

• Foreign key must “look like” related primary key– Same number of fields

• Field names don’t have to match

– Data types must match

Referential Integrity

• Ensures that data is consistent– Value in foreign key must exist in related

primary key– Prevents “orphans”, records on many side

without a valid “parent”

• Creates limits on both tables– Can’t enter a row in the many side with a

foreign key that doesn’t exist– Can’t remove a row on the one side if there are

related rows in the many side

Cascade Update/Delete

• Can implement referential integrity to help manage changes automatically

• Cascade update passes changes to primary key values to the related rows on the many side– If student ID is changed in students, change

the student ID in related rows in Enrollments to match the new value

• Cascade delete deletes related rows from the many table when a row from the one-side is deleted– If a student is deleted from Students table,

delete related rows in Enrollments

Normalizing a database

• Process of organizing data in database to:– Reduce redundancy: don’t repeat values

• Some repetition is needed for primary keys/foreign keys

– Reduce inconsistent dependencies: changing one value shouldn’t require a change to a second value• Rather than store Price, Quantity, Total Price (which is

Price * Quantity), store Price, Quantity and calculate Total Price when needed

• Helps to ensure each table is about one thing

Using Normalization

• Different degrees of normalization are referred to as “forms”

• Database designer determines how far to normalize– Most relational databases are in 3rd normal

form– Some “de-normalization” is common

• Each higher level of normalization leads to more tables with fewer columns

• More joins in queries are required to make data useful, understandable

First Normal Form

• First Normal Form is most basic level• Each row/column combination has only

one value– Address should be broken up to Street, City,

State, Zip

• Eliminate repeating groups– Don’t have multiple phone number columns in

a Students table

First Normal Form Example

• Example: Instead of using a single field for all items purchased, each item is unique, as is quantity

OrderIDCustomerIDOrderDateItems Purchased

Not Normalized OrderIDCustomerIDOrderDateItemIDQuantityItemName

1st Normal Form

Second Normal Form

• Remove fields that are not fully dependent on the key, and place in separate table(s)– Each row should be about just one thing– Listing the grades a student receives doesn’t

help describe a student – grades belong in a different table

Second Normal Form Example

• Example: ItemID is not dependent on the Customer and OrderID; it is dependent on OrderID (an order can include many different things)

OrderIDCustomerIDOrderDateItemIDQuantityItemName

1st Normal Form

OrderIDCustomerIDOrderDate

2nd Normal Form

OrderIDItemIDQuantityItemName

Third Normal Form

• All non-key columns are mutually independent– A change in one field does not require a

change in another field in the table (i.e. no calculations)

• All fields contribute to describing the key (making the record unique)

• There are limits to how far to go:– A change in city could require a change in

state and zip code– Need to either add many small tables or some

level of not being normalized

Third Normal Form Example

• If the ItemID that’s part of an order changes, that means that item name should change too; break out Products into it’s own table

OrderIDCustomerIDOrderDate

2nd Normal Form

OrderIDItemIDQuantityItemName

OrderIDCustomerIDOrderDate

3rd Normal Form

OrderIDItemIDQuantity

ItemIDItemName

Normalization Summary

• A change in one field should not require change in another field in the table– No calculations

• All fields help describe the key– Each record is unique– Each table stores information about one

“thing”