[Up] | Reference for unit 'DbCtrls' (#lcl)

## HowToUseDataAwareControls - Hints for accessing databases

HowToUseDataAwareControls - Hints for accessing databases. A set of Data-Aware components is provided, to make it easier to access information held in databases.

Before using Data-Aware components, it is necessary to make a connection with a database, and this should be done either using a TSQLConnection or a TDBf component, depending on the type of database that is being used. In any case, the components for connection with the database need to be found in $Lazdir/components/sqldb/, $Lazdir/components/tdbf/ or whatever other source is appropriate. They need to be added using the Components Menu Item of the IDE, and compiled ready for adding. The IDE needs to be re-built to include the new components.

To make the connection with the database: first place a DataBase Connection on the Form (invisible at run-time, so put it anywhere convenient); set its properties to connect the user (with name and password) to the appropriate server, database and table (the connection component should deal with all the user-authentication issues, opening and closing the database etc). Then put on a TDataSource from the Data Access tab of the Component Palette, and perhaps items such as TSQLTransaction and TSQLQuery, depending on the particular database you are using. The various database components need to be linked to each other, for example by setting the Transaction property of a database connector to the appropriate transaction component, and linking the DataBase property of the Transaction component back to the database connector; link the DataSource's DataSet property to the appropriate SQLQuery or other set of data, and its Transaction property to the transaction component

Each Data-Aware control (on the Data Controls tab of the Component Palette) has the usual set of properties to determine its shape and size, position, colour and font etc. But there are also special properties to define the data to be displayed or modified. These include the DataSource property which must be set to link to the defined set of data, and often a Field component to determine which of the columns in the database is to be displayed.

Most of the components operate on a single Row or record of the database, but the TDBGrid is capable of displaying and operating on multiple rows and columns, in a StringGrid format, allowing the user to get an overview of what is in the database. The DBGrid will display the contents of the DataSet even at design time, provided the various components are enabled or rendered active; this allows the designer to ensure that all the connections to the database have been correctly made.

The TDBNavigator component is used in conjunction with the Data-Aware components. It consists of a series of buttons to allow browsing through the DataSet, displaying the first, previous, next or last record (or Row), inserting or deleting records, committing the changes back to the database or refreshing the DataSet from the database.

The way database programming works is typically as follows:

- a database is opened for access by the user (Database Connection)
- a command is issued (eg by TSQLQuery) to select or extract some or all of the records which are then stored locally in the DataSet
- the contents of the DataSet are acted on by the Data-Aware components, displaying one Row or record at a time from selected Columns, or perhaps the whole DataSet in a DBGrid, editing records, inserting or deleting records, or using the data to perform statistical tests or calculations
- at intervals usually prescribed by the user (sometimes after every change, sometimes after a whole series of changes), the contents of the local DataSet are sent back to the database, causing an update of the database's contents. This process is referred to as 'Commit' or 'Post' or sometimes as 'Update'

The programmer can determine which events are associated with a Commit action by supplying suitable event handlers for OnClick, OnChange, OnFinishedEdit etc.

The following table shows the approximate correspondence between conventional and data-aware control classes

Standard Control | Data-Aware Control | Comment
--- | --- | ---
TCalendar | TDBCalendar | Select a date from pop-up calendar
TCheckBox | TDBCheckBox | Check a box to show an option has been selected
TComboBox | TDBComboBox | Select an option from a list and return it (or some custom text) in the main edit box
TEdit | TDBEdit | Single-line text editing box
TGroupBox | TDBGroupBox | A container for several related controls
TImage | TDBImage | A Picture
TListBox | TDBListBox | A list of string items
TMemo | TDBMemo | Multiple-line text editing box
TRadioGroup | TDBRadioGroup | A group of mutually exclusive Radio Buttons; only one at a time may be checked and selected. Note there is no data-aware equivalent of the isolated TRadioButton
TText | TDBText | A simple text-string, usually read-only
TStringGrid | TdbGrid | A grid to display information in Rows and Columns

Standard Controls can, of course, be used to display information from a database, but they lack the direct link with the DataSet and its Navigation aids, so the programmer must write explicit code to extract data from the DataSet and put it in the Standard Control

The latest version of this document can be found at lazarus-ccr.sourceforge.net .