CSE 103 Day 15: Database Design Students: Do NOT log in Read Day 2 in your textbook Your BT feedback is up—check after class, save questions for then. Others: Please save your work and log out by 10:10. http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Day 13/14 Review Day 13: Nested queries in MySQL Write subqueries to find data needed before we can answer the overall problem Compare a field to a subquery returning exactly one record & field by using =, >, <, etc. In a FROM clause, subqueries can be used as virtual tables we name via AS Day 14 BT: Comments/feedback on your BT? "It was too hard" doesn't count How long were queries taking to run? http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Database terminology review Entity A particular type of object about which data is stored Tables are used to represent entities Record A group of data about a specific object, usually represented by one row in one of the tables Tables contain records about specific objects of that same type Field/Attribute An individual type of information about an object Stored in columns in the tables http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Database Design What questions do I want the database to answer? What data do I need to answer these questions? How are these data related to one another? http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Guiding Principles Control data Redundancy Maintain data Consistency Don't (redundantly) store data in two places (twice) Maintain data Consistency Don't store conflicting data Provide data Integrity Only allow legal values; all records must have primary keys Maintain data Independence Changing data in one place does not “break” the DB in other places http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Database Example: Electronic Rolodex What information about your friends would you like a rolodex to provide? What data is in the SBC phone book white pages? What data isn't available in the SBC phone book that you would like to have in your personal phone book? How would you arrange this data in a database table?  http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Database Example: Electronic Rolodex, cont’d What questions do you want to be able to answer about your friends? What items of data do we need in our contact directory in order to answer such questions? What should you do when you can't get some of the data? For example, you don't know all of your friends' addresses. What should you do with people who have more than one phone number: a cell phone number, a pager number, a fax number? http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Data types Examine the list of data types on page 2-5 of the text Within your group, reach a consensus on the datatype assigned to each piece of data in your design Note: this data will be made available in the Day 15 AFS space after we have agreed on types as a class http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Design View for a Table Start Access and create a new database called contacts.mdb Create a new table in design view, and enter the field names and data types (as agreed) Use descriptive field names close to those discussed in class so that you don't need any additional information in the Description box NEVER USE SPACES IN FIELD NAMES! Try to follow a consistent convention, e.g., all lower-case, first capital, underscores between words, etc. Pick your own but BE CONSISTENT! Don't save the table yet http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Saving the Table Use a naming convention: To tell the difference between tables and queries, we use tbl_ and qry_ Not required—some people don't use any convention, but we recommend you use one Following the naming convention, call the table tbl_People Access will prompt you about a primary key. Answer NO for now and continue to the next slide. http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Primary Keys What does the primary key do? What is a reasonable primary key for this table? (see next slide) http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Types of Primary Keys Existing data: Multiple cell: Autonumber: e.g., Social Security #, PID, filename, etc. One item that uniquely identifies an object Multiple cell: e.g., ActorID + MovieID = Role (list of Actors in Movies); TextbookID + AuthorID = BookAuthor (books’ authors) The combination of all fields uniquely identifies an object: same actor can’t be in same movie, but one actor can be in multiple movies, and one movie may have multiple actors Autonumber: Last resort, used when nothing else works well The DBMS assigns a new unique # when you insert a row Wastes space if you could find a key among existing data Doesn't help prevent duplicate entries (can always insert; get new key every time, even if all other data is the same) http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Closing & data entry Set the primary key, save, and close the table Open it in datasheet view and add information about yourself Remember that we don't ever save the database itself All data changes are live Changes to table or query designs have to be saved http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Better design, first steps: Properties of data What pieces of information does every person have: Exactly one of Exactly zero or one of Any number between zero and two (or more) What pieces of information might be shared between multiple people? http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103

Homework Check the Homework link Enter 15 records into your contact directory (Suggest you use classmates right after class!) Practice identifying primary keys Read Day 10 in the textbook on the design of multiple-table databases Think about how we could improve the design of the contacts database (see questions on previous slide) http://www.cse.msu.edu/~cse103 U:/msu/course/cse/103