Databases MIS 21

Some database terminology  Database: integrated collection of data  Database Management System (DBMS): environment that provides mechanisms for storage and access of data  Relational Database Model: views a database as a collection of relations or tables where each table stores information on an entity

Tables, attributes and columns  A table represents an entity in a system  The rows of the table represent records or instances of the entity  The columns of the table represent the entity’s attributes or characteristics  How do you determine a system’s entities? Spot the nouns (people, places, things) in the system

Example: The STUDENT table IDLastNameFirstNameYearQPI CruzJuan SmithJohn CruzMaria SantosLinda43.0 Columns/Attributes There are four rows/records in this example

Column types/domains  Each column in a table has an associated type indicating the possible values for the attribute  Most common types Strings: CHAR, VARCHAR Numbers: INTEGER, DOUBLE, NUMERIC Date and time: DATE, TIME, TIMESTAMP

Primary key, foreign key, relationships  Primary key: column or attribute that uniquely determines a row in the table STUDENT table example: ID is the primary key  Foreign key: attribute in one table that refers to a record in another table Used to store relationships (relationships are verbs in the system) Example: a department code in the EMPLOYEE table refers to the DEPARTMENT table, signifying that the employee belongs to the department

Relationship example EmpIDNameSalaryDeptCode 123CRUZ, JUAN HR 222SANTOS, MARIA HR 545MATIPID, PETE ACCT DeptCodeDeptName ACCTACCOUNTING HRPERSONNEL MAINTMAINTENANCE EMPLOYEE DEPARTMENT Employee SANTOS, MARIA works for the PERSONNEL department

Structured Query Language  SQL: Structured Query Language Also called “SeQueL” Standard underlying language for database definition, manipulation, and query  SQL statements SELECT (query) INSERT UPDATE DELETE Others (Data Definition)

The SELECT statement  A query that returns a table  Simplest form: SELECT column1, column2, … FROM table SELECT EmpID, Salary FROM EMPLOYEE  List all records, all columns SELECT * FROM table SELECT * FROM DEPARTMENT  List particular record/s SELECT * FROM table WHERE criteria SELECT * FROM STUDENT WHERE year=3

The WHERE clause  Indicates selection criteria or condition against which records on the table will be checked  Contains operators such as, = =, <> and LIKE <> means not equal LIKE performs a pattern match with strings ( _ matches a single character, % matches several characters) Condition may have logical operators (AND, OR, NOT)  Note the importance of the column type when specifying the condition Literal strings (e.g., CHAR) need to be delimited by quotes or double quotes

ORDER BY clause  Can indicate ORDER BY in the SELECT statement to specify ghe order in which the records will be listed  Syntax: append ORDER BY col1,col2,… to the SELECT statement  Indicate ORDER BY col1,col2,… DESC to list records in descending order

More examples  SELECT Name FROM EMPLOYEE WHERE SALARY >  SELECT FirstName,Lastname FROM STUDENT WHERE LastName LIKE ‘S%’  SELECT * FROM STUDENT WHERE year <> 2  SELECT * FROM STUDENT WHERE year=3 ORDER BY LastName, FirstName  SELECT Name FROM EMPLOYEE ORDER BY Salary

SELECT on multiple tables  FROM clause can contain multiple tables  Use when processing relationships  Tables are joined; indicate join condition in WHERE clause  Example: SELECT Name, DeptName FROM EMPLOYEE, DEPARTMENT WHERE EMPLOYEE.DeptCode = DEPARTMENT.DeptCode AND Salary >10000 returns all names of employees (and the name of the departments they belong to) who make more than 10000

The INSERT Statement  Inserts a record into a table  Syntax: INSERT INTO table VALUES(val1,val2,…) INSERT INTO table(col1,col2,…) VALUES(val1,val2,…)  Examples: INSERT INTO DEPARTMENT VALUES(“CS”, “COMPUTER SCIENCE”) INSERT INTO EMPLOYEE(EmpID, Salary) VALUES(143, )

The UPDATE Statement  Updates an existing record  Syntax: UPDATE table SET col1=expr1,col2=expr2,… UPDATE table SET col1=expr1,col2=expr2,… WHERE criteria  Examples (updates all records) UPDATE EMPLOYEE SET Salary=Salary+100 (updates some record/s) UPDATE STUDENT SET QPI = 4.0 WHERE FirstName = “Matalino”

The DELETE Statement  Deletes records from a table  Syntax: DELETE FROM table WHERE criteria (warning: without a WHERE clause, all records are deleted)  Example DELETE FROM EMPLOYEE WHERE DeptCode=“CS”

Summary  A relational database consists of tables that store records of a system  The database is manipulated through SQL, the underlying query and manipulation language of relational databases  SELECT statements carry out queries  INSERT, UPDATE, and DELETE statements affect the database