Eastern Mediterranean University School of Computing and Technology. ITEC319 Rapid Application Development

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1 Eastern Mediterranean University School of Computing and Technology ITEC319 Rapid Application Development Database Operations The simplest type of database is the local database. A local database is a database that resides on a single machine. This type of database would probably consist of few tables. Only one user that uses the program accesses to this database. Any edits made to the database are written directly to the database. MS Access is the most known local database. 2 1

2 Database Operations Another way a database can be implemented is as a client/server database. The database itself is stored and maintained on a file server (server side). One or more users (clients) have access to the database. The users of this type of database are likely to be spread across a network. There might be more than one attempt to access the database at the same time. This isn't a problem with client/server databases because the server knows how to handle all the problems of simultaneous database access. 3 Database Operations in Delphi To enable access to local databases and to client/server databases, Delphi provides the Borland Database Engine (BDE). The BDE is a collection of DLLs and utilities that enables access to a variety of databases. The figure below shows the relationship between your application, the BDE and the database. 4 2

3 Database Operations in Delphi Naturally, database formats and APIs vary widely. For this reason the BDE comes with a set of drivers that enables your application to talk to several different types of databases. These drivers translate high-level database commands (such as open or post) into commands specific to a particular database type. This permits your application to connect to a database without needing to know the specifics of how that database works. All versions of Delphi come with drivers enabling you to connect to most of the local and client/server databases. 5 Database Operations in Delphi The BDE uses an alias to access a particular database. A BDE alias is a set of parameters that describes a database connection. In its simplest form, an alias tells the BDE which type of driver to use and the location of the database files on disk. After you create an alias for your database, you can use that alias to select the database in your Delphi programs. 6 3

4 Database Components in Delphi The VCL database components fall into two categories: nonvisual data access components and visual data-aware components. The nonvisual data access components provide the mechanism that enables you to get the data and the visual data-aware components enable you to view and edit the data. 7 Database Components in Delphi The data access components are derived from the TDataSet class and include TTable, TQuery and TStoredProc. The visual data-aware components include TDBEdit, TDBListBox, TDBGrid, TDBNavigator and more. These components work much like the standard edit, list box and grid components except that they are tied to a particular table or field in a table. By editing one of the data-aware components, you are actually editing the underlying database as well. 8 4

5 Database Components in Delphi Interestingly, these two component groups cannot talk directly to each other. Instead, the TDataSource component acts as an intermediary layer between the TDataSet components and the visual data-aware components. 9 Database Components in Delphi Database tables are often displayed in spreadsheet format. The column headers across the top indicate the field names. Each row in the table contains a complete record. Figure below shows such a database table displayed in grid (or table) format. The cursor points to the record that will be read if data is requested and the record that will be updated if any edits are made. The cursor is moved when a user browses the database, inserts records, deletes records and so on. 10 5

6 Database Components in Delphi A collection of data returned by a database is called a dataset. A dataset can be more than just the data contained in a table. A dataset can be the results of a query containing data acquired from many tables. For example, let's say you have a database containing names and addresses of your customers, their orders, and the details of each order. This data might be contained in tables named Clients, Orders and Order Details. Now let's say you request the details of the last 10 orders placed by Company X. You might receive a dataset containing information from the Clients table, the Orders table and the Order Details table. Although the data comes from several different sources, it is presented to you as a single dataset. 11 Database Components in Delphi The following table shows the most important properties of a TTable component. Property Name Active Filter Filtered Database Name Tablename Description Name of the component. Opens the dataset when set to true and closes it when set to false. An expression that determines which records a data set contains. When true, the table is filtered based on either the filter property or the OnFilterRecord event. When false, the entire dataset is returned. The name of the database that is currently being used. The name of the database table 12 6

7 Database Operations in Delphi A practical example: 1. First of all, database has to be created. But as a first example, a demo database will be used in RAD Studio 2009 named as DBDEMOS. 2. Place a Table, Datasource and DBgrid components on the form. 3. Firstly select Table component and set some of its properties as follows: DatabaseName should be selected from the drop down list TableName should be set 4. Table component acts as a dataset for the datasource component. So, dataset property of datasource component should be set to Table1. 5. Finally, DBGrid component should be connected to the datasource component. The datasource property of DBGrid component should be set to Datasource1. 6. Without writing any line of code, the data within the database can be shown on the DBGrid component. 7. For this purpose, the Active property of the Table component should be set to True. 13 Filtering a Table in Delphi A common need of a database application is to filter a table. Filters are primarily used on local databases. Filters are rarely used with client/server databases; instead, a SQL query is used to achieve the same effect that filters have on local databases. Consider that you might have a table with thousands of records, but you are interested in displaying or working on only a small subset of the table. 14 7

8 Filtering a Table in Delphi Filters in the Table component are handled in two ways: through the Filter property and the OnFilterRecord event. Filtered property determines whether the table is filtered. If Filtered is True, the table will apply the filter currently in force (either the contents of the Filter property or the results of the OnFilterRecord event). If Filtered is False, the contents of the Filter property are ignored and the OnFilterRecord event is never generated. 15 Filtering a Table in Delphi In order to use the Filter property, a field name, a logical operator and a value should be written into this property. A filter might be written as follows: Total>150 Price<=12 This statement filters the table dataset by comparing the values in the total field with 150. Those entries which is higher than 150, will be removed from the DBGrid. 16 8

9 Filtering a Table in Delphi Filters can also use the keywords AND, OR or NOT: Total>150 and Price<=12 The field name and the logical operators (AND, OR or NOT) are not case sensitive. For example, the following two filter statements are identical: TOTAL>150 AND PRICE<=12 total>150 and price<=12 17 Filtering a Table in Delphi In the case of searching for text, the FilterOptions property determines how the filter will be applied. This property is a set that can contain either or both focaseinsensitive or fonopartialcompare. By default this property is an empty set, which means that filters will be case sensitive and will enable partial comparisons. For example, the filter string that results in a dataset containing all records in which the LastName field begins with the letter M is shown below. LastName = M* 18 9

10 Filtering a Table in Delphi The following operators can be used in filter statements: Operator Description < Less than > Greater than = Equal to <> Not equal to <= Less than or equal to >= Greater than or equal to ( ) Used to specify the evaluation order of compound expressions [ ] Used around field names containing spaces AND, OR, NOT Logical Operators 19 Filtering a Table in Delphi An example: 1. Select the table component in the last developed project. 2. Type the following lines into the filter property and see the output in your program. Total>150 ID=1 Total>100 and Price>=12 3. In order to apply the filter, filtered property should be set to True

11 Filtering a Table in Delphi Filtering can also be applied at run time, through the code. For example insert an edit box and a button onto the form and type the following line of code to the onclick event of the button. Table1.Filter := 'total>'+edit1.text; Table1.Filtered := True; So, the filter will be applied with the value entered into the edit box. 21 Filtering a Table in Delphi The other way to filter a table is done through the OnFilterRecord event of the Table component. The last example can also be implemented using OnFilterRecord event instead of Filter property. procedure TForm1.Table1FilterRecord(DataSet: TDataSet; var Accept: Boolean); var filterstr : integer; begin filterstr := Table1.FieldByName('total').Value; Accept := (filterstr > strtoint(edit1.text)); end; 22 11

12 Filtering a Table in Delphi The key element here is the Accept parameter. The OnFilterRecord event is called once for every row in the table. You should set condition for the Accept parameter to True for any rows that you want to show. The preceding code sets Accept to True for any rows in which the Total field is greater than the value entered into the edit box. Although, using OnFilterRecord means more work, it's also much more powerful than filtering with just the Filter property