CS 101 – Access notes

Databases (Microsoft Access)

• 4 parts of a database

• database design– Try to understand the ideas behind database

design, not just the mechanics.

Access vs. Excel

• Excel emphasizes numbers

• Access emphasizes relationships

• some overlap

• Access is perfect for finding common info from 2 separate tables

Purpose of Access

• It’s a “database management system”

• Record keeping for a business– Requires a plan– one file, with many components

• Keep track of … so that:– Find cross-references– Generate reports

Database format

Usually four parts:

• Tables (at least 1 is required)

• Queries

• Forms

• Reports

Navigating a table

• A database table looks like a spreadsheet!

• Record = row

• Field = column

• Two ways to view:– Design view, to start– Datasheet view, to enter data

• Primary Key!

Example questions

• In a stock portfolio:– Which stocks have lost money?– When did I buy those stocks?

• Which class is easier: CS 22 or CS 25?– Check grades in each class– Check grades for students who’ve taken both

• Among customers who bought a new generator last year, what have they bought since?

Relieve tedium

• Suppose we want to maintain info on orders.

• Better to have two tables than one!

Order# Name City State Product Qty.

101 J. Miller Miami FL Laptop 5

102 G. Novak Tampa FL Printer 3

103 G. Novak Tampa FL DVD-RW 50

104 G. Novak Tampa FL Keyboard 1

105 J. Miller Miami FL Lamp 4

106 A. Bik San Diego CA Chair 6

107 T. Lee Buffalo NY Fan 3

Need multiple tables

• Ex. Librarian wants to contact graduating seniors who have overdue books.

• Need multiple tables:– Student info table

• Student #, name, year

– Book table• ISBN, title, author, …

– Transaction or “check out” table• Transaction #, ISBN, student #, Due date

Relationships

• When you have 2+ tables, there is almost always a relationship

• They share one field in common.– Can you tell what it is?

• Ex. Customers & Orders• Ex. Students & Class roster• Ex. Publishers & books

Animal hospital

• Keep track of customers, pets, visits

• Each gets its own table– What fields for each table?– Relationships

• What else does a database need?

Relationships

Fields in your table

• Anticipate questions– Age? store birth date– GPA? store credits and quality points– What year? store date of admission

• Store data in its smallest parts (e.g. address)

• Calculated fields don’t belong in table!

Table review

• We want to “manage” information:– Organize, insert, delete, retrieve

• To organize we…– Create a table (or “set”, “class”)

consisting of records (or “objects”)each having fields (or “attributes”)

• Usually we’ll want 2+ related tables.

Fields in your table

• Anticipate questions– Age? store birth date– GPA? store credits and quality points– What year? store date of admission

• Store data in its smallest parts (e.g. address)

• Calculated fields don’t belong in table!

Queries

• Usually we ask about info from 2+ tables.• By default, a query will perform an operation

called a Cartesian Product, which gives all possible combinations.

• Ex. Name and City tables:

Name

Bob

Mary

Ken

City

Miami

Pittsburgh

Cartesian Product

• Given 2 sets, find all possible ordered pairs.– Analogously for more than 2 sets.

• Great example: choosing a menu.– Appetizer– Entrée– Dessert

• Unfortunately, most DB queries are not like this! We get too many results.

Relationships

• We want to tell Access that there is a relationship between the tables, so we can create meaningful query.

• One-to-many is most common– “Each city has one or more employees.”– Now, query will return 3 results instead of 6:

Miami BobPittsburgh Mary

Ken

One-to-One

• Can be useful if some information is confidential.

• What if we didn’t have any relationship?

Empl # Name Position

101 Smith Welder

102 Jones carpenter

Empl # Salary

102 18,000

101 17,000

1-1 Query

• When you combine tables that have a 1-1 relationship: Access will look for fields that are the same, and use this as a filter.– In previous example, we’ll have 2 results

instead of 4. Employee 101’s informationEmployee 102’s information

– Let’s look at another example.

• What happens when we “join” these 1-1 tables?

First name Last name Position City

Bob Fuller accountant Chicago

Bob Daniels cashier Greenville

Bob Daniels accountant Indianapolis

Alice Andrews gardener Greenville

First name Last name Salary Birthday

Bob Daniels 51,000 7/1/67

Elena Carlson 21,000 9/1/89

Alice Andrews 81,000 8/1/78

First Last Position City Salary Birthday

Bob Daniels cashier Greenville 51,000 7/1/67

Bob Daniels accountant Indianapolis 51,000 7/1/67

Alice Andrews gardener Greenville 81,000 8/1/78

First name Last name Position City

Bob Fuller accountant Chicago

Bob Daniels cashier Greenville

Bob Daniels accountant Indianapolis

Alice Andrews gardener Greenville

First name Last name Salary Birthday

Bob Daniels 51,000 7/1/67

Elena Carlson 21,000 9/1/89

Alice Andrews 81,000 8/1/78

One-many Relationships• Referential integrity

– Keep related records consistent

– Cascade delete: allow deletion of “one”

– Cascade update: allow update of “one”• For example, changing someone’s CustomerID.

Many-to-many

• Ex. Customers to products

• Implement as 2 one-to-one

• “Order details” table

• Think of possible queries based on the 5 tables given in handout.

Other queries

Besides ordinary “select” queries:

• Total – special case: also do subtotals • Parameter – prompt user to tailor the result

• Action – modify underlying table– Make, delete from, append to, update

• Crosstab – 2-D subtotal– Ex. $, by species and month!