Querying Database

ISYS 363

Basic Query Language Operations

• Selection

• Projection

• Join

• Aggregates: Max, Min, Sum, Avg, Count– Totals and SubTotals– GroupBy

• Calculated fields

Selection

• Selection operation retrieves records that satisfy user’s criteria.

Criteria

• >, >=, <, <=, =, <>

• Range: BETWEEN 1/1/03 AND 12/31/03

• Wildcard:– ? – match any one character

• “K?NG”

– * - Match any number of characters• “C*”

Projection

• Projection operation defines a vertical subset of a table and retrieves only the specified fields.

• Example: Customer table: CID, Cname, City, Rating

– Create a query to show CID, Cname, Rating– Create a query to show CID, Cname, City

Join

• The two tables must have common attributes:– Key and foreign key.

• Combines two tables to form a new table where records of the two tables are combined if the common attributes have the same value.

Join Example

Faculty File: FID FnameF1 ChaoF2 SmithF5 Boxer

Student File:SID Sname FIDS1 Peter F1S2 Paul F2S3 Smith F1

Faculty Join Student =

Join Example

Student File:SID Sname FIDS1 Peter

F1S2 Paul

F2S3 Smith

F1

StudentCourse File:SIDCIDS1ISYS263S1Acct101S3ISYS363S2ISYS263S2Fin350S2Acct101

Course File:CID Cname

UnitsISYS263 IS Intro

3ISYS363 MIS Intro

3Acct101 accounting

3Fin350 Finance Intro

3

(Student Join StudentCourse) Join Course

Aggregate Functions

• Max, Min, Sum, Count, Avg

• QueryTools/Totals

• Ex. Employee: EmpID,Ename, Sex, Race, Birthdate, Hiredate, Salary– How many employees in this company?– What is the overall average salary?

Aggregates by Group

– Compute the average salary by race.– Compute the average salary by sex.– Compute the average salary by Race and

Sex.– How many employees in each race group?

University Database

ERD Notations

Student Account

Faculty Course

Has1 1

EnrollM MAdvise

M

1

SIDSname Major SID Balance

FID Fname CID CnameUnitsPhone

Grade

GPA

Query Examples• Q1: Display students’ ID, name and account

balance who owe university more than $2000.

• Q2: Display student’s ID, name and total units.

• Q3: Find students taking at least 9 units and display their ID, Name and total units.

• Q4: Display CID, Cname, SID, Sname• Q5: Display CID, Cname, number of

students in each course.• Q6: Display faculty’s name and phone if the

faculty advises at least three students.

Top n query: Example Top 3 GPA1. Sort GPA in descending order2. From Query/Design view, click Property Sheet and select Top Value

Order Processing Database

Query Examples

– Find customers live in San Francisco.– Produce a customer report that shows CID, Cname,

and Rating.– Number of customers in each city

• City, NumbeOfCustomers

– Produce a report that shows the number of orders for each customer:

• CID, Cname, TotalNumberOfOrders

– Total sales by product– Total amount for each order– MIS report– Criteria applied to subtotal

Calculated Fields

• Rename a field:– NewName:OldName

• Define a calculated field:– Tax:salary*.15– Age:Year(Now()) – Year(DOB)– IIF function

Sorting

• One field sorting

• Two fields sorting

Other Queries

• Update query:– Query tools/Design/Query type/Update

• Delete query

• Parameter query

• CrossTab query– Crosstab row

Access Tools for Import/Export

• Import from Excel:– The first row of Excel’s list should contain field

names– External Data/Import/Excel

• Export to Excel:– External Data/Export/Excel

Open Database Connectivity (ODBC)

• Provide a standard to retrieve data from a database.

• It manages one or more "database drivers“ that enables the communication between database and applications.

• To access a database, we use ODBC facilities to define a ODBC data source name for the database.