Introduction to Databases Queries CS 146

Sample Database: CANDY_CUSTOMER CANDY_PURCHASE CANDY_CUST_TYPE CANDY_PRODUCT

Basic Database Vocabulary  Field: column of similar data values  Record: row of related fields  Table: set of related rows Field Record

Sidenote: Database History  All pre-1960’s systems used file-based data  First database: Apollo project  Goal: to not store duplicate data in multiple locations  Used a hierarchical structure  Created relationships using pointers  Pointer: hardware address

Example Hierarchical Database Student ID Student LastName Student FirstName Student MI Pointers * to Course Data 5000NelsonAmberS 5001HernandezJosephP 5002MyersStephenR UniversityStudent CourseIDCourse Name Course Title 100MIS 290Intro. to Database Applications 101MIS 304Fundamentals of Business Programming 102MIS 310Systems Analysis & Design UniversityCourse *Pointer – physical location (as a number) to the start of the referenced data

Problems with Hierarchical Databases  Relationships are all one-way; to go the other way, you must create a new set of pointers  Pointers are hardware/hard drive-specific  VERY hard to move to new hardware  Applications must be custom-written  Usually in COBOL

Relational Databases  Circa 1972  E.J. Codd  “Normalizing” relations  Store data items only once  With the exception that foreign keys can be duplicated  Stores data in a tabular format  Creates relationships through sharing key fields

Key Fields  Primary key: uniquely identifies a record InstructorIDInstructor LastName Instructor FirstName 1BlackGreg 2McIntyreKaren 3SarinNaj UniversityInstructor Primary keys

 What is the primary key of each table in the CANDY database?  How can you tell if a field is a primary key? Class Discussion

Special Types of Primary Keys  Composite PK: made by combining 2 or more fields to create a unique identifier o Consider the CANDY_PURCHASE table…  Surrogate PK: ID generated by the DBMS solely as a unique identifier

Duplication Considerations  When data values appear multiple times, there is duplication  Problems:  Space  Data becomes inconsistent over time StudentIDStudent LastName Student FirstName StudentMIAdvisorLast Name AdvisorFirst Name 5000NelsonAmberSBlackAnne 5001HernandezJosephPBlackAnne 5002MyersStephenRSarinNaj UniversityStudent

Key Fields (continued)  Foreign key  Field that is a primary key in another table  Serves to create a relationship StudentIDStudent LastName Student FirstName StudentMIAdvisorID 5000NelsonAmberS1 5001HernandezJosephP1 5002MyersStephenR3 UniversityStudent Foreign keys InstructorIDInstructor LastName Instructor FirstName 1BlackGreg 2McIntyreKaren 3SarinNaj UniversityInstructor Primary keys

 What are the foreign keys in the CANDY database?  Does a table HAVE to have foreign keys?  When would you use them?  How can you tell if a field is a foreign key? Class Discussion

 Every record has to have a non-NULL and unique PK value  Every FK value must be defined as a PK in its parent table Rules for Relational Database Tables (non-negotiable)

Structure of a Database DB Data DBMS Client Workstations Database Server

Database Structure  A database consists of multiple user accounts  Your area in the database is called your user schema  Identified by your username and password  Each user schema contains database objects that you create  Tables  Views  Stored programs  Etc.

Query Browser  Example: Oracle SQL developer

 Query: command to perform an operation on a database object  Create  Insert  Modify  View  Delete  Structured Query Language (SQL)  Standard query language for relational databases Database Queries

MySQL Query Browser Type query:Click Execute:

Query Conventions  Not case-sensitive  Convention: reserved words in all-caps, user-supplied values (table names, field names, etc.) in lower-case letters  Queries can span multiple lines  Semi-colon marks the end of a line

Retrieving Data From a Single Table Syntax: SELECT column1, column2, … FROM schema.tablename WHERE search_condition SELECT candycust_id, candycust_name FROM candy_customer WHERE cust_id = 1

Retrieving all Fields or Records  To retrieve all fields in the table: use the "*" wildcard character  To retrieve all records in a table: omit the search condition SELECT * FROM tablename WHERE search_condition

How many fields and how many records will the following query retrieve? A. 7 fields and 14 records B. 14 fields and 7 records C. 7 fields and 9 records D. None of the above SELECT * FROM candy_purchase;

Search Conditions  General format: FieldName Operator TargetValue  Operators: =,, =, <> or !=  Examples:  PROD_ID = 1  POUNDS > 5  STATUS != 'PAID'

Search Conditions (continued)  Number: just type the number  Text string:  Case-sensitive  Enclose in single quotes  Date:  Enter as a text string in ‘dd-mon-yy' format: WHERE purch_date = ‘28-Oct-04’

Which records will the following query retrieve? A. Purch_id values 2, 3, 5, 7, 8 B. Purch_id values 2, 7, 8, 9 C. Purch_id values 2, 7, 8 D. None of the above SELECT * FROM candy_purchase WHERE pounds >= 5

Which records will the following query retrieve? A. Purch_id values 1, 2, 3, 4, 6, 8 B. Purch_id values 5, 7, 9 C. All purch_id records will be returned D. No purch_id records will be returned E. An error will occur SELECT * FROM candy_purchase WHERE status = 'Paid'

Searching for NULL Values  NULL: undefined  Search conditions for NULL and non-NULL values: WHERE column_name IS NULL WHERE column_name IS NOT NULL

Combining Multiple Search Conditions  AND: query only retrieves records for which both conditions are true WHERE Condition1 AND Condition2  OR: query retrieves records for which either condition is true WHERE Condition1 OR Condition2

Using AND and OR in Search Conditions  Every expression must be well-formed:  Do this:  Not this: WHERE purch_date > ‘28-Oct-04' AND purch_date < ‘1-Nov-04’ WHERE purch_date > ’28-Oct-04' AND < ‘1-Nov-04'

Which records will the following query retrieve? A. Purch_id values 4, 6, 8 B. Purch_id values 1, 2, 3, 4, 5, 9, 12, 13 C. Purch_id values 1, 2, 3, 4, 5, 9, 10, 11, 12, 13 D. None of the above SELECT * FROM candy_purchase WHERE delivery_date IS NULL AND status = 'PAID'

Which records will the following query retrieve? A. Purch_id values 1, 2, 3, 4, 6, 7, 8, 12 B. Purch_id values 1, 2, 3, 4, 12 C. Purch_id values 1, 2, 3, 4, 5, 9, 12, 13 D. None of the above SELECT * FROM candy_purchase WHERE delivery_date = NOT NULL AND status = 'PAID'