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Transcript
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 F1 F2 F5 Student File: SID S1 S2 S3 Fname Chao Smith Boxer Sname Peter Paul Smith Faculty Join Student = FID F1 F2 F1 Join Example Student File: SID S1 S2 S3 Sname Peter Paul Smith StudentCourse File: SID CID S1 ISYS263 S1 Acct101 S3 ISYS363 S2 ISYS263 S2 Fin350 S2 Acct101 FID F1 F2 F1 Course File: CID Cname ISYS263 IS Intro ISYS363 MIS Intro Acct101 accounting Fin350 Finance Intro Units 3 3 3 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 SID Sname GPA Major 1 Student M M Advise Has Enroll SID 1 Balance Account M Grade 1 Faculty FID Fname Course Phone CID Units Cname 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 GPA 1. Sort GPA in descending order 2. 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.