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: – 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: MiamiBob PittsburghMary Ken

One-to-One Can be useful if some information is confidential. What if we didn’t have any relationship? Empl #NamePosition 101SmithWelder 102Jonescarpenter Empl #Salary 10218, ,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 information Employee 102’s information –Let’s look at another example.

What happens when we “join” these 1-1 tables? First nameLast namePositionCity BobFulleraccountantChicago BobDanielscashierGreenville BobDanielsaccountantIndianapolis AliceAndrewsgardenerGreenville First nameLast nameSalaryBirthday BobDaniels51,0007/1/67 ElenaCarlson21,0009/1/89 AliceAndrews81,0008/1/78

FirstLastPositionCitySalaryBirthday BobDanielscashierGreenville51,0007/1/67 BobDanielsaccountantIndianapolis51,0007/1/67 AliceAndrewsgardenerGreenville81,0008/1/78 First nameLast namePositionCity BobFulleraccountantChicago BobDanielscashierGreenville BobDanielsaccountantIndianapolis AliceAndrewsgardenerGreenville First nameLast nameSalaryBirthday BobDaniels51,0007/1/67 ElenaCarlson21,0009/1/89 AliceAndrews81,0008/1/78

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.