Querying Database ISYS 363. Basic Query Language Operations Selection Projection Join Aggregates:...
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Querying Database
ISYS 363
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Basic Query Language Operations
• Selection
• Projection
• Join
• Aggregates: Max, Min, Sum, Avg, Count– Totals and SubTotals– GroupBy
• Calculated fields
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Selection
• Selection operation retrieves records that satisfy user’s criteria.
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Criteria
• >, >=, <, <=, =, <>
• Range: BETWEEN 1/1/03 AND 12/31/03
• Wildcard:– ? – match any one character
• “K?NG”
– * - Match any number of characters• “C*”
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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
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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.
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Join Example
Faculty File: FID FnameF1 ChaoF2 SmithF5 Boxer
Student File:SID Sname FIDS1 Peter F1S2 Paul F2S3 Smith F1
Faculty Join Student =
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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
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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?
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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?
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University Database
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ERD Notations
Student Account
Faculty Course
Has1 1
EnrollM MAdvise
M
1
SIDSname Major SID Balance
FID Fname CID CnameUnitsPhone
Grade
GPA
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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.
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Top n query: Example Top 3 GPA1. Sort GPA in descending order2. From Query/Design view, click Property Sheet and select Top Value
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Order Processing Database
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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
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Calculated Fields
• Rename a field:– NewName:OldName
• Define a calculated field:– Tax:salary*.15– Age:Year(Now()) – Year(DOB)– IIF function
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Sorting
• One field sorting
• Two fields sorting
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Other Queries
• Update query:– Query tools/Design/Query type/Update
• Delete query
• Parameter query
• CrossTab query– Crosstab row
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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
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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.