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Agenda – 10/23/2012

Provide answers for HW#7 if requested. Present SQL View database object.

Database for Sample Code in this Handout

Emp

PK EmpID

LastName FirstName Address City State Zip HireDate OfficePhone BillingRateFK1 managerIDFK2 jobtitleID

TimeWorked

PK,FK1 EmpIDPK StartWork

FK3 ContractIDFK2 WorkTypeID Amount

Work

PK WorkTypeID

StdBillRate Description

allocates

is allocated

to

is of

describes

Contract

PK ContractID

FK1 ClientID DateSigned DateDue EstHours ContractTypeID

Is assigned

to

manages

JobTitle

PK jobtitleID

description

has

Client

PK clientID

Name Address City State Zip Phone Fax ContactName

Is with

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What is a SQL View?

A “virtual” table. A set of SQL statements that creates a result table which can

be accessed by other SQL statements. Similar to a function in another programming language, except

that the end-product of the view is a named result table. A database object.

The code for a view is stored in the database. A view contains no data of its own. A view relies on the data in the base tables used to create the

view. A set of stored SQL code.

Stores code; not data.

Let’s say we frequently look at all the information about time in our database and we always convert the time to hours and we always like to include the name of the employee who worked

the time as well as the description of the type of work performed:

SELECT ISNULL(emp.empid,tw.empid) empid,

emp.lastname,

tw.contractid,

tw.startwork,

tw.worktypeid,

work.description,

ISNULL(amount/60,0) FROM empFULL OUTER JOIN timeworked tw ON emp.empid = tw.empidLEFT OUTER JOIN workON work.worktypeid = tw.worktypeidORDER BY 1;

Create a view out of the code

CREATE VIEW vEmptime ASSELECT ISNULL(emp.empid,tw.empid) empid,

emp.lastname,

tw.contractid,

tw.startwork,

tw.worktypeid,

work.description,

ISNULL(amount/60,0) hoursworkedFROM empFULL OUTER JOIN timeworked tw ON emp.empid = tw.empidLEFT OUTER JOIN workON work.worktypeid = tw.worktypeid;

Must eliminate the ORDER BY

clause

Must add the CREATE VIEW

statement

Must alias any field with a calculation,

aggregation or function

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How is the view used via SQL?

Views are used just as tables are used in SQL. In this example, the join is predefined, so it is easier to write queries to access the data.

SELECT * FROM VEmpTimeORDER BY empid;

SELECT *FROM VEmpTimeWHERE hoursworked = 0;

SELECT lastnameFROM VEmpTime;

Can a view be joined with tables?

SELECT VEmpTime.empid,VEmpTime.lastname,VEmpTime.startwork,VEmpTime.contractid,client.name

FROM VEmpTimeINNER JOIN contractON contract.contractid = VEmpTime.contractidINNER JOIN clientON contract.clientid = client.clientidORDER BY VEmpTime.empid,

VEmpTime.contractid desc;

Must have a “shared” column to access the data between tables and view – same as using

a foreign key between tables.

Last week in lab, we discussed a correlated sub-query

SELECT empID,lastname,empOuter.jobtitleID,description,billingrate "Employee Billing Rate",(SELECT AVG(billingrate)

FROM emp empSelect WHERE empOuter.jobtitleID = empSelect.jobtitleID)

"Average Billing Rate" FROM emp empOuterLEFT OUTER JOIN jobtitleON empOuter.jobtitleID = jobtitle.jobtitleIDWHERE billingrate >

(SELECT AVG(billingrate) FROM emp empInner WHERE empOuter.jobtitleID = empInner.jobtitleID)

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Group Functions and Joins are Complex

Must have all non-group attributes that are in the SELECT list also in the GROUP BY statement.

Difficult to do a group function of a group function. Examples:

The maximum of the sum of hours.

The minimum of a count of products.

Joining multiple tables can yield full or partial cartesian products making it difficult to trouble-shoot the SQL code.

Create a “view” database object

CREATE VIEW vAvgRateByTitleASSELECT jobtitleID,

AVG(billingrate) AverageBillRateFROM empGROUP BY jobtitleID;

Remember: when using a VIEW, any derived column (calculated, aggregate and/or with a

SQL function) must have an alias

Look at the VIEW results and use the VIEW in another query

SELECT *FROM vAvgRateByTitle;

SELECT emp.empid,emp.lastname,emp.billingrate,emp.jobtitleid,vAvgRateByTitle.AverageBillRate

FROM empLEFT JOIN vAvgRateByTitleON emp.jobtitleid = vAvgRateByTitle.jobtitleidWHERE emp.billingrate > AverageBillRate;

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So, what’s the big deal?

Views allow you to break down difficult queries into smaller pieces for easier design, coding and debugging.

Views allow you to create a layer of abstraction between the data structure and the user or programmer allowing greater security. Programmers do not know the structure of the base tables. Less

risk of fraud. Users can see “pre-joined” tables for their queries. Users don’t have to understand the complexity of the actual

database. No one sees data that is secure (salary, for example).

Views allow you to access the results of aggregate functions more easily.

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Examples of more complex questions

Which employee worked the most total hours in September?

What is the description of the work type with the most time in the time table and how much time was reported for that work type description?

For which contract have we spent the most time?

What is the name of the client for whom we worked the most time during the month of August in the current year?

Which contracts have had more time worked than the estimated time?

Which contracts have more than 25% time worked than the estimated time?

Let’s find out which employee worked the most hours in September

Where do the columns come from (which table)?

What is the basic logic of the query?

What is the simplest component that can be written to accomplish the basic logic?

Write the basic logic in pseudocode

SELECT employee stuffFROM timeworkedWHERE sum(timeworked) for month of September =

max(sum(timeworked)) for month of September

This code doesn’t work!! It is just written to get an understanding of the basic logic

necessary to accomplish the query.

Use a view to summarize data

CREATE VIEW vEmpHours ASSELECT empID,

month(startwork) MonthWork,sum(amount/60) TotalHours

FROM timeworkedGROUP BY empid,

month(startwork)

Maybe create another view if you want...

CREATE VIEW vEmpHoursSept ASSELECT *FROM vEmphoursWHERE monthwork = 9;

Use the view to work on the basic logic

SELECT * FROM vEmpHoursSept WHERE totalhours = (SELECT MAX(totalhours) FROM vEmpHoursSept)

Now add the “extra” stuff, piece at a time...

SELECT vEHS.empid,firstname + ' ' + lastname "Employee Name",officephone,totalhours

FROM vEmpHoursSept vEHSLEFT OUTER JOIN emp on vEHS.empid = emp.empidWHERE totalhours = (SELECT MAX(totalhours)

from vempHourssept)

Finish it up!

SELECT vEHS.empid,firstname + ' ' + lastname "Employee Name",officephone,description,totalhours

FROM vEmpHoursSept vEHSLEFT OUTER JOIN emp on vEHS.empid = emp.empidLEFT OUTER JOIN jobtitle jt on jt.jobtitleid = emp.jobtitleid WHERE totalhours =

(SELECT MAX(totalhours) from vempHourssept)

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Let’s do another example!

I want to know information about the contract for which we have worked the most hours. I

want the output to look like this:

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Let’s do it in pieces

Identify the underlying tables or calculations that will be used to

find and/or calculate the columns in the result table.

Define the basic “logic” of the query. Use pseudocode to help

make that definition.

Don’t focus on the columns that are “pretty,” i.e. provide information to the

user of the query but don’t affect the basic logic of the query.

The basic “logic” usually relies on some kind of relational condition.

If you don’t understand the basic “logic” of the query, then play

around with SQL to figure it out.

First, write a query that summarizes the amount of time (divided by 60 so

that the sum is in hours) in the TIMEWORKED table by contractid.

Second, use that query to figure out how to find the MAX of the SUM.

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Conclusion: Uses of views

Views are used to: Provide easier access to data. Enhance security. Lessen the visible complexity of the database.

Views are usually created by the DBA for a defined workgroup of people. Programmers. Users. Users in a specific functional area.