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Computer Science 101 Web Access to Databases Overview of Web Access to Databases
Why web access to database? • Provides platform independent, remote access to the database – Viewing data – Gathering data – Updating data
Client-Server Technologies • Server: A program that provides services to other programs. It stands ready for requests and when it gets a request, it provides the service. • Client: A program requesting a service of a server program. It makes a request, gets the service, and makes use of it.
Web Browsers and Servers • Web server - This is a program that runs on the internet host computer (server). It takes requests for web pages from clients and delivers the pages back to the client. • Web browser - This is a program that runs on your local PC (client). It allows you to request web pages from an internet host.
HTML and HTTP • Hyper. Text Markup Language - Allows “marking up” a document with tags specifying appearance and structure. • Hyper. Text Transfer Protocol - Protocol used for browsers to communicate with web servers. Basically, this consists of “requests” from the browser and “responses” from the server.
Typical HTTP Request HTTP Response <HTML> <B> This is a web page </B> <IMAGE> Picture</IMAGE> </HTML> Browser This is a web page Web Server HTTP Request http: //website/some. html Browser interprets HTML and displays page
Typical HTML Request • Client Side (Browser) • Server Side – Issues request for HTML page – Reads request from client – Receives response HTML page – Finds page on server – Interprets HTML and creates web page – Displays web page – Work is done here – Sends response page to client – Essentially a file server
Server Side Programs • Here we have compiled programs that run on the server. Examples could be Java Servlets, ASP. NET languages such as C#
Typical Server-Side Request Runs script or program <HTML> on the server <% server script or program %> </HTML> <H 1> Stars </H 1> <B> John Wayne <BR> Meg Ryan </B> </HTML> Sends response to client Gets Page Web Server HTTP Request http: //stars. aspx Client Stars John Wayne Meg. Ryan Browser interprets HTML and displays page
Typical Server-Side Request • Client Side (Browser) – Issues request for HTML page – Receives response HTML page – Interprets HTML and creates web page – Displays web page • Server Side – Reads request from client – Finds page on server – Runs server-side program needed – Alters HTML file – Sends response page to client
Web Access of Database • The database resides on the server. • Web pages with scripts (or calls to programs) allow the user to send database requests to the server. • The server accesses the database to honor the requests. • Results can be returned on an html page. • Actions can take place on the database.
So, what are the pieces we need? • Browser software on user machines • A machine for the server • Web server software on server • Database management system on server • Scripting/programming language for accessing the database – something supported by the web server
Web Applications: 3 -Tier Architecture User Query User Interface SQL Query Application Logic - Program Results -HTML Query results Browser Web Server with Local Machine ASP. NET (C#) Database Access Database
The Big Picture – User Interface • User Interface contains web components for user input or for displaying information returned from the database. • Two types of components that we will use for displaying data in our examples will be Grid. Views and Drop. Down. Lists. • Grid. Views provide row and column displays. They can be configured for really nice appearance, paging, etc. • These components can have associated with them a data source – essentially a database query. We’ll keep it simple.
The Big Picture – Middle Tier • The middle tier has program code to get users input, connect to the database, present request to the database, get results from the database, send results on to the interface. • This is where the server side programming code would come in.
The Big Picture – Database • Database receives requests in form of SQL statements or in form of request for stored query, performs the query and returns results to middle tier. • You are already expert at this part, correct?
Simple model – retrieving and displaying data from the database • For now we are going to consider examples where we want the web page just to display information resulting from a query to the database. • We’ll see that Visual Web Developer is very helpful with setting up connections to database, “binding” web components to data sources, etc.
Example 1 – Faculty Information • As a first example, let’s have a web page with a data grid displaying the faculty information – Last. Name, First. Name, Phone extension, and email address. • So we would – Start Visual Web Developer – Open our website – Create a new file (webform)
Example 1 – Faculty Information (cont. ) • After placing any heading or whatever on the page, we drag a Grid. View onto the page from the Data Section of the Tool. Box.
Example 1 - Choosing the data source for the Grid. View New data source Browse for database Access Database
Example 1 - Setting up the query
Example 1 - Testing the query
Example 1 - View the web page
Example 1 - Modification • Now let’s modify Example 1 so that it displays information only for the advisors – department chairs who are not advisors will not appear. • This only requires modifying the query associated with the grid view. • To get to the query editor – Click on the grid view – Then click on the triangle in the upper right corner of the grid view. – Click on Configure data source
Example 1 – Modifying the query • There is nothing in the Faculty table to indicate whether or not a person is an advisor of one of our class members. • We need to use the Student table as well as the Faculty table. SELECT DISTINCT F. Last. Name, F. First. Name, Phone, Email FROM Faculty F, Students S WHERE Faculty. ID = Advisor. ID
Example 1 – Modifying the query
Example 2 - Majors • To begin, we’ll have a drop down list with the various majors listed. • The process is the same as before, put the drop down list on the page and configure the data source.
Data source for drop down list
Example 2 – Viewing page with drop down list
Example 2 – Grid. View with students • Now let’s modify Example 2 by adding a grid view that displays the names, class year, city, state and advisor fields for the students having the major selected from the drop down list. • To have the selection in the drop down list take place, we need to have a way to force the page to “post back” to the server. • We’ll use a button for now. It does nothing except that clicking a button causes the form to be “posted” to the server.
Example 2 – Add Grid. View and Button
Example 2 – Data Source for Grid View
Example 2 – Testing the web page
Example 2 – Adding advisors last name • Now let’s modify the query so that we see the advisor’s last name rather than the advisor’s ID number.
Example 2 – Testing the page
Example 2 – Removing the button • There is a property of drop down lists called Auto. Post. Back. • The default value for this is false. • If this value is set to true, the page will post back to the server whenever an item in the drop down list is selected. In Example 2, we can set this property to true and remove the button. • In many cases, we do not want that to happen since we may want to make several selection before having the action take place – make selections and then click button.
Example 3 – Stored query • Often it is easier to develop and test queries within the database itself as you did in lab. • If we save these queries, they can be used in configuring a data source just like tables.
Example 3 – Data source