CS 101 – Nov. 9

Text software issues (continued)– Readability– Fonts

• Database concepts – Relationships

– Queries

Readability

• First, need to measure:– Words per sentence– Syllables per word

• How to Write Plain English by Flesch:206.835 – 84.6*spw– 1.015*wps

90’s = fifth grade

30’s = college level

Fonts

• Font = style of printing– Typeface– Point size– Ex. Times 10, Helvetica 12

• Fonts can be distinguished by– Sarif or sans sarif– Proportional or non-proportional

Font types

• Sarif = has feet

• Sans sarif = no feet– Look at: i, h, n, r, … (but not t!)

• Proportional = width of characters changes• Non-proportional = all have same width

Access review

• Purpose: Manage data• Parts of a database

– If your database has only 1 table, you are better off using Excel

• Relationship between tables?– None

– One-to-one

– One-to-many (most common)

– Many-to-many

Table design

• What fields to we need?– 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

Relationship summary

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

Example

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.