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

ID LastName FirstName Year QPI

081111 Cruz Juan 2 3.5

072222 Smith John 3 2.2

089999 Cruz Maria 3 4.0

077676 Santos Linda 4 3.0

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

EmpID Name Salary DeptCode

123 CRUZ, JUAN 12345.00 HR

222 SANTOS, MARIA 11111.00 HR

545 MATIPID, PETE 30000.00 ACCT

DeptCode DeptName

ACCT ACCOUNTING

HR PERSONNEL

MAINT MAINTENANCE

EMPLOYEE

DEPARTMENT

Employee SANTOS, MARIAworks for thePERSONNEL 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 columnsSELECT * FROM table SELECT * FROM DEPARTMENT

List particular record/sSELECT * 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: appendORDER BY col1,col2,…to the SELECT statement

Indicate ORDER BY col1,col2,… DESCto list records in descending order

More examples SELECT Name FROM EMPLOYEE

WHERE SALARY > 12000 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 DEPARTMENTVALUES(“CS”, “COMPUTER SCIENCE”)

INSERT INTO EMPLOYEE(EmpID, Salary)VALUES(143,10000.00)

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