Work in progress
- 1 Introduction
- 2 Available Components
- 3 Explanation of the used components
- 4 Our program
- 5 Sources
In the Database tutorial we saw a first attempt to connect to a MySQL server. We did not use any components, either visual or non-visual at that time. This page will explain how to connect to a MySQL server in a (perhaps) easier way.
If you look in the $Lazarus/components/mysql directory you will find two lazarus packages. There is a package for MySQL version 3.2x (mysql3laz.lpk) and one for MySQL version 4.x (mysql4laz.lpk). If you did not do so yet, install the appropriate package, depending on the server version. Remember that it is not possible to use the package for version 3 to connect to a server running version 4 or vice versa. When you've installed this package you will see a new tab in the component palet, called MySQL. On this tab there are two components, TMySQLDatabase and TMySQLDataset.
NB. Before trying to install this components in Lazarus be sure that you have appropiated MySQL client libraries or else Lazarus may not start after install.
Another possibility is the installation of the package in the $Lazarus/components/sqldb directory. In this directory you see a package file called sqldblaz.lpk. You need to install this package and the mysql4connlaz.lpk form the $Lazarus/components/sqldb/mysql directory. The first package contains some general components used for all databases. These component are TSQLTransaction and TSQLQuery and can be found on the new SQLdb tab. After installation of the mysql4connlaz.lpk you will find a third component on the SQLdb tab called TMySQLConnection (depicted by a dolphin).
If you do not know how to install components / packages, have a look at this page for an "Install Howto". As the SQLdb components are the more general and can be used for other databases just by replacing the TMySQLConnection with for instance a TIBConnection, we will develop a program with the SQLdb components.
NB. (And this is rather important!) SQLdb components can use only MySQL client libraries version 4.0, other versions as 3.x, 4.x, etc. Will not work! however applications should be able to connect to a MySQL server of any version.
Explanation of the used components
The TMySQLConnection is used to store parameters to connect to the database server. It enables you to set the host to connect to and the userid and password to use in the connection. Another property of the TMySQLConnection is used to indicate the database you want to use. The 'LoginPrompt' is not functional yet, so make sure that next to the HostName and DatabaseName the UserName and Password properties have values as well before you try to open the connection. Be sure to use a TSQLTransaction as well and connect it to your MySQLConnection be setting the Transaction property, or you will not be able to open a SQLQuery.
A SQLTransaction is needed for some internal housekeeping. A SQLTransaction is automatically activated when you open a dataset using it. When you want to close the SQLConnection however, you first have to make sure the SQLTransaction is not active.
TSQLQuery is used to execute SQLstatements on the server. You can retrieve data by setting the SQL to some SELECT statement and call the Open method. Or you can manipulate data by issuing some an INSERT, DELETE or UPDATE statement. In the latter case you should not use the Open method but the ExecSQL method.
A datasource provides the connection between the visible data aware components like DBEdit, DBGrid and a dataset. It makes the data available for the data aware components to display. A datasource can only be connected to a single dataset at a time but there can be several data aware components connected.
A DBGrid can be used to present the data retrieved by a Dataset. The DBGrid needs a datasource to connect to a dataset. When the dataset is opened, the DBgrid will automatically be populated with the data.
The main form
We will use the same main screen and build all functionality from scratch :) As you will see there is a lot less to take care of, because the components really take away all the hard stuff! So lets start by making a screen that looks like this.
From the SQLdb-tab place a TMySQLConnection, a TSQLTransaction and a TSQLQuery on this form. Don't change the default names given to this components. We have to link these components together so they can do their job. So the following properties have to be set:
The Transaction-property of SQLQuery1 will automatically be set if you have set the Transaction property of MySQLConnection1 first. When you set this, you will notice that SQLTransaction1.Database has been set to MySQLConnection1.
As you can see in the screen dump the only buttons available on start of the program are "Connect to server" and "Exit". For the other buttons to work we need more information so these are disabled. We could decide to disable "Connect to Server" as well until the information for the host, username and password has been given. I decided against this because our user might think: "Nothing seems possible, so let's hit exit." :)
Before I start giving you any code I would like to stress that there should be more exception handling in the code. Critical sections should be placed in
try ... finally
try ... except
Connect to a server
The first thing we have to do is get connected to our server. As when connecting we don't know what databases are available on the server we will ask for a list of databases on connecting. However there is one catch, to make the connection we have to enter a valid DatabaseName in the properties of the MySQLConnection. You will see in the code that I am using the "mysql" database. This database is used by mysql for some housekeeping so it will always be there.
procedure TFormTryMySQL.ConnectButtonClick(Sender: TObject); begin // Check if we have an active connection. If so, let's close it. if MySQLConnection1.Connected then CloseConnection(Sender); // Set the connection parameters. MySQLConnection1.HostName := HostEdit.Text; MySQLConnection1.UserName := UserEdit.Text; MySQLConnection1.Password := PasswdEdit.Text; MySQLConnection1.DatabaseName := 'mysql'; // MySQL is allways there! ShowString('Opening a connection to server: ' + HostEdit.Text); MySQLConnection1.Open; // First lets get a list of available databases. if MySQLConnection1.Connected then begin ShowString('Connected to server: ' + HostEdit.Text); ShowString('Retrieving list of available databases.'); SQLQuery1.SQL.Text := 'show databases'; SQLQuery1.Open; while not SQLQuery1.EOF do begin DatabaseComboBox.Items.Add(SQLQuery1.Fields.AsString); SQLQuery1.Next; end; SQLQuery1.Close; ShowString('List of databases received!'); end; end;
The first thing we do is check to see if we are connected to a server, if we are then we call a private method "CloseConnection". In this method some more housekeeping is done. like disabling buttons and clearing comboboxes and listboxes. Then we set the necessary parameters to connect to server.
- Throughout our program you may see calls to ShowString. This method adds a line to the memo on our form which acts like a kind of log.
With the parameters set, we can connect to the server. This is done by calling
In a proper application one would place this in an exception handling construct to present a friendly message to the user if the connection failed. When we are connected we want to get a list of databases from the server. To get data from the server a TSQLQuery is used. The SQL property is used to store the SQL-statement send to the server. MySQL knows the "SHOW DATABASES" command to get the list of databases. So after we have set the SQL-text, we call
The result set of a SQLQuery can be examined through the fields property. As you can see we iterate through the records be calling
When we have added all available databases to our combobox, we close the SQLQuery again.
Selecting a database
If the user selects a database in the DatabaseComboBox we enable the "Select Database" button. In the OnClick event of this button we set the DatabaseName of MySQLConnection1, and request a list of tables. The last statement of this procedure enables the "Open Query" Button, so the user can enter a query in the "Command" Editbox and have it send to the server.
procedure TFormTryMySQL.SelectDBButtonClick(Sender: TObject); begin // A database has been selected so lets get the tables in it. CloseConnection(Sender); if DatabaseComboBox.ItemIndex <> -1 then begin with DatabaseComboBox do MySQLConnection1.DatabaseName := Items[ItemIndex]; ShowString('Retreiving list of tables'); SQLQuery1.SQL.Text := 'show tables'; SQLQuery1.Open; while not SQLQuery1.EOF do begin TableComboBox.Items.Add(SQLQuery1.Fields.AsString); SQLQuery1.Next; end; SQLQuery1.Close; ShowString('List of tables received'); end; OpenQueryButton.Enabled := True; end;
MySQL has a special command to get a list of tables, comparable to getting the list of databases, "show tables". The result of this query is handled in the same way as the list of databases and all the tables are added to the TableComboBox. You might wonder why we do not open the connection again before opening the query? Well, this is done automatically (if necessary) when we activate the SQLQuery.
Fields in a table
In MySQL you can again use a form of "SHOW" to get the fields in a table. In this case "SHOW COLUMNS FROM <tablename>". If the user picks a table from the TableComboBox the OnChangeEvent of this ComboBox is triggered which fills the FieldListbox.
procedure TFormTryMySQL.TableComboBoxChange(Sender: TObject); begin FieldListBox.Clear; SQLQuery1.SQL.Text := 'show columns from ' + TableComboBox.Text; SQLQuery1.Open; while not SQLQuery1.EOF do begin FieldListBox.Items.Add(SQLQuery1.Fields.AsString); SQLQuery1.Next; end; SQLQuery1.Close; end;
As well as the names of the fields, the result set contains information on the type of field, if the field is a key, if nulls are allowed and some more.
Showing the data
Well as we said we would use components to get connected to the database, lets use some components to show the data as well. We will use a second form to show a grid with the data requested by the user. This form will be shown when the user typed a SQL command in the "Command" editbox and afterwards clicks the "Open Query" button. This is the OnClick event:
procedure TFormTryMySQL.OpenQueryButtonClick(Sender: TObject); begin ShowQueryForm := TShowQueryForm.Create(self); ShowQueryForm.Datasource1.DataSet := SQLQuery1; SQLQuery1.SQL.Text := CommandEdit.Text; SQLQuery1.Open; ShowQueryForm.ShowModal; ShowQueryForm.Free; SQLQuery1.Close; end;
The ShowQueryForm looks like this:
and contains a
The button is placed on the panel. What happens in the "Open Query" OnClick is this. First we create an instance of TShowQueryForm. Secondly we set the DataSet property of the DataSource to our SQLQuery1. Then we set the SQLQuery SQL command to what the user entered in the "Command" editbox and open it. Then the ShowQueryForm is shown modally, this means that it will have the focus of our application until it is closed. When it is closed, we "free" it and close SQLQuery1 again.