ODBCConn

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The ODBCConn unit implements an SQLdb connection to ODBC data sources.

Contents

Overview

ODBC

ODBC (Open Database Connectivity) is a technology that allows one to connect to a whole variety of databases using a single API, the ODBC API.

There are ODBC implementations for various platforms and there are drivers for most Database Management Systems. Official ODBC documentation can be found at the MSDN ODBC site.

TODBCConnection

FreePascal ships with ODBC headers; they are in the odbcsql and odbcsqldyn units. TODBCConnection is an TSQLConnection descendant providing a nice OOP wrapper for ODBC using the SQLdb framework.

In Lazarus, you can find the TODBCConnection component on the SQLdb component tab. You can also use the TODBCConnection component in your code by adding ODBCConn to your uses clause.

sqldbcomponents.png

What has been implemented:

  • executing queries and retrieving result sets
  • most field types, including blobs
  • query parameters (string and integer types)
  • preparing queries
  • UpdateIndexDefs (so you can use ApplyUpdates)

What is left to be implemented:

  • proper transaction support; currently each connection corresponds to one transaction
  • some field types
    • SQL_TYPE_UTC* (these are mentioned in the ODBC docs, but seem not to be used in implementations)
    • SQL_INTERVAL_* (what would be the corresponding TFieldType?)
    • SQL_GUID (TGUIDField was not implemented, until recently)

Why use ODBC?

FreePascal ships with components for connecting to several databases, such as MySQL, PostGreSQL, Firebird, Microsoft SQL Server (since 2.6.1), Oracle, etc.

For those databases missing from the list, like MS Access and perhaps Microsoft SQL Server, ODBC is an acceptable and well established solution. (VB.net/C# developers are recommended to use SQLClient rather than OLEDB or ODBC for the ultimate in performance, but here in Lazarus ODBC will work adequately and reliably)

The TODBCConnection component was developed originally to circumvent the strict MySQL license for applications that are not GPLed or do not obey MySQL AB's FLOSS exception.

Installing ODBC and ODBC Drivers

Before you can connect to your database using ODBC, you need to install

  • an ODBC Driver Manager
  • an ODBC driver specific to the DBMS you want to connect to

This section will give a brief overview of the steps involved. However, please consult the relevant documentation for a definitive reference.

The ODBC Driver Manager

Windows

Windows has an ODBC Driver Manager built in, which allows DSNs to be set up, and other configuration. It is found in the Control Panel, in later Windows versions with categorised sections it was moved into the Administrative Tools area. Or you may simply click the Start button and enter ODBC into the Run box (in Windows 7 this appears as "Search programs and files").

You can, of course, create a desktop shortcut to the ODBC dialog if you find yourself using it frequently.

WinXPODBCDataSourceAdministratorDrivers.png

Unices

Two popular ODBC Driver Managers for Unix-based platforms are unixODBC and iODBC. ODBCConn is known to work with unixODBC; iODBC compatibility still has to be tested.

Debian

For Debian, you can install the unixodbc package:

aptitude install unixodbc
aptitude install unixodbc-bin # if you want some GUI tools

The odbcsqldyn unit, and hence odbcconn, will search for a file called libodbc.so. It will not accept a file named like libodbc.so.1 or libodbc.so.1.0.0. Debian's unixodbc package does not create a symlink with the name /usr/lib/libodbc.so; you must either

  • create the link yourself: ln -s libodbc.so.1 /usr/lib/libodbc.so, or
  • install the unixodbc-dev package, which does create the symlink.

If you installed the unixodbc-bin package, you can run the ODBCConfig program to configure ODBC drivers and DSNs.

ODBCDataSourceAdministratorAbout.PNG

Ubuntu

For Ubuntu, follow the instruction for Debian. Note: the unixodbc-bin package might not be available from the default package repository.

ODBC Drivers

TODO

Connecting to an ODBC data source

The parameters for connecting to an ODBC data source are described in a connection string. This is a string of the form NAME=VALUE;NAME=VALUE.

TODBCConnection provides a wrapper around this connection string. Some of its properties are mapped to name-value pairs in the connection string, and custom parameters can be specified in the Params property (which is a TStrings).

Before going to the details of this wrapper, you must first have a basic understanding of how an ODBC data source is identified.

ODBC connections via DSN

An ODBC driver manager provides alternative ways to make DSN shortcuts for a set of parameters:

  • DSN (DataSource Name): a system or user defined data source, identified by a (unique) name. DSNs can be configured using the ODBC Data Source Administrator or by manually editing the odbc.ini file (or registry).
  • File DSN: a file which contains connection parameters. An ODBC Data Source Administrator usually allows you to create File DSNs from the GUI.

The parameters in a DSN or File DSN can always be combined with additional parameters in the connection string, for example to specify a password.

By setting up a DSN the connection details can be verified to work within the manager dialog, and then the named DSN is all that is needed to use the connection later. The connection details are therefore decoupled from your application, as only the DSN name is used in your software - the DSN acting as a go-between.

An advantage to using a named DSN is that you may easily switch between databases using the ODBC Manager, without changing your code. This is useful in commercial development to test for bugs using a number of sets of your clients' data.

Without a DSN

You may also connect via ODBC without using a DSN, simply supplying all the details in the Connection String that you would otherwise set up in a DSN (driver,server,database,login details). This more direct method avoids the need to set up a DSN when installing your application on a new machine.

The ODBC specification defines a few parameters that can be used in a connection string:

  • Two special parameters, DSN and FILEDSN, allow one to select a set of pre-defined parameters, as described above.
  • The DRIVER specifies which ODBC driver to use. Obviously, this is a very important parameter.
  • The UID and PWD parameters are used to supply a username and password.

All other parameters are driver dependent. Please refer to the documentation of the specific driver to learn more about available parameters and their names.

TODBCConnection properties

The following table describes the mapping of TODBCConnection properties to ODBC connection string parameters:

Property Type Connection string parameter
Driver string DRIVER
DatabaseName string DSN, not to something like DATABASENAME, which is not part of the ODBC standard
FileDSN string FILEDSN
Password string PWD
UserName string UID
HostName string none; there is no corresponding parameter in the ODBC standard
Params TStrings Use this to specify custom parameters. Each item must be of the form NAME=VALUE.

One important parameter that can be used is AUTOCOMMIT which determines if SQL statements are directly executed (without possibility of a rollback) (default setting, or set explicitly using AUTOCOMMIT=1) or whether you need to manually call StartTransaction and Commit, CommitRetaining or Rollback (set using AUTOCOMMIT=0)

Note that TODBCConnection will take care of escaping parameter values in the connection string (when required).

The LoginPrompt boolean property is not implemented yet. It would require finding the correct window handle, so a driver can show a GUI dialog to specify parameters. Note that this is not controlled by the connection string, but rather by the last parameter to the ODBC API function SQLDriverConnect.

Examples

In this section, examples are given of connecting to certain DBMSs using their specific ODBC drivers.

Connecting to Excel

Probably works on Windows only.

To avoid errors like: Could not start transaction! ODBC error details: LastReturnCode: SQL_ERROR; Record 1: SqlState: HYC00; NativeError: 106; Message: [Microsoft][ODBC Excel Driver]Optional feature not implemented ;

... you'll have to enable autocommit, something like

ODBCConnection1.Params.Add('AUTOCOMMIT=1');

Connecting to MySQL

For a reference of supported parameters by the MyODBC driver, see [1].

The driver name differs a bit depending on the platform and MyODBC version; examples are :

 { properties of connection object, i.e. conn: TODBCConnection; }
 conn.Driver := 'MySQL'; // (Unix) 
 conn.Driver := 'MySQL ODBC 3.51 Driver';  // (Windows)
 conn.Driver := 'MySQL Connector/ODBC v5'; // (Windows)
 // note: driver name doesn't need {} like it does in SQL Connection Strings elsewhere

Other parameters (when not using a DSN) :

 conn.UserName := 'myUsername';
 conn.Password := 'myPassword';
 // conn.Params (TStrings) set using .add method :
 conn.Params.Add('SERVER=example.com');
 conn.Params.Add('PORT=3306');
 conn.Params.Add('DATABASE=myDatabase');

Connecting to MS Access

Please see MS Access.

Connecting to Microsoft SQL Server

Note-icon.png

Note: Newer versions than FPC 2.6.0 and Lazarus versions using those support a direct SQLDB connection to MS SQL Server; see MSSQL

See http://www.connectionstrings.com/sql-server#p7 for more details on connection strings.

Note-icon.png

Note: If you use MS SQL Server ODBC connections, you may need to set Query.PacketRecords to -1 in order to avoid "could not retrieve primary key metadata" errors.

Named DSN

On Windows, you can use something like this with an existing named DSN:

 { properties of connection object, i.e. conn: TODBCConnection; }
 conn.DatabaseName := '<Your_DSN_Name>';
 { Leave all other conn. properties empty }

DSN-less

Without a DSN:

 { properties of connection object, i.e. conn: TODBCConnection; }
 conn.Driver := 'SQL Server';
 // note: driver name doesn't need {} like it does in SQL Connection Strings elsewhere
 { Leave Hostname and DatabaseName properties empty }
 { set conn.Params (TStrings) : }
 conn.Params.Add('Database=<yourdatabasename>');
 conn.Params.Add('Server=.\SQLEXPRESS');
 conn.Params.Add('Trusted_Connection=Yes'); // or 'Integrated Security=SSPI'

Note: In this example, we connect to the local machine (server=.), on the instance SQLEXPRESS. You can use a hostname instead of ., and you can omit the instance to connect to the default instance. See MS documentation for details.

Using a trusted connection (also known as integrated security or SSPI) means you login using your Windows user credentials. You can omit the Trusted_Connection line, but then you need to specify user ID (Uid=...) and password (Pwd=...).

DSN example

The example code below selects the rows from a MS SQL Server table called 'journal_entries' and displays all the values of column 'journal_entry' in a Memo control called Memo1.

// uses  ODBCConn, sqldb;
procedure TForm1.Button1Click(Sender: TObject);
var
  S: String;
  conn : TODBCConnection; // uses ODBCConn
  query: TSQLQuery;       // uses sqldb
  transaction: TSQLTransaction;   // uses sqldb
 
begin
  conn := TODBCCOnnection.Create(nil);
  query := TSQLQuery.Create(nil);
  transaction := TSQLTransaction.Create(nil);
  try
    try
      conn.DatabaseName := 'sqlserverdsn'; {replace this with your DSN, if you use any}
      conn.UserName:= 'sa'; //replace with your user name
      conn.Password:= 'thepassword'; //replace with your password
      // You can override the properties defined in the DSN, e.g.
      //conn.Params.Add('Database=some_other_db');
 
      conn.Transaction := transaction;
      query.DataBase := conn;
      { To avoid "could not retrieve primary key metadata":
      - either use query.UsePrimaryKeyAsKey:=false
      or (preferred)
      - query.PacketRecords to -1
      If you are using SQL Server, you could alternatively enable MARS in your SQL Native Client ODBC driver.
      See http://bugs.freepascal.org/view.php?id=13241 }
      query.PacketRecords:=-1; //retrieve all data at once
      query.SQL.Text := 'select journal_entry from journal_entries';
      query.Open;
      S := '';
      while not query.EOF do
      begin
        S := S + query.FieldByName('journal_entry').AsString + #13#10;
        query.Next;
      end;
    finally
      query.Free;
      conn.Free;
      transaction.Free;
    end;
  except
    on E: Exception do
      ShowMessage(E.message);
  end;
  Memo1.Text:= S;
end;

In fact it is possible to use conn.Params for all parts of the connection string, if we modify the above - this time showing the SQL Server version in a pop-up message :

  conn.Params.Add('Driver=SQL Server');
  conn.Params.Add('Server=OURBIGSERVER');
  conn.Params.Add('Database=TestDataBase');
  conn.Params.Add('Integrated Security=SSPI'); // use Windows logon credentials
  // no other conn. properties set, apart from conn.Transaction := transaction;
 
  query.DataBase := conn;
  query.SQL.Text := 'select @@version';
  query.Open;
  showmessage(query.Fields[0].AsString); // Fields are numbered starting from 0

Functions are supported - e.g. 'SELECT * FROM MyTestFunc()' - but there may be issues with Stored Procedures that return recordsets. For example, if 'EXEC MyStoredProcedure' returns data rows (verified in SQL Server Management Studio or with the SqlCmd DOS command: C:\>sqlcmd -S MyServer -E -d MyDataBase -Q "exec MySP") then query.open will give an error : "Cannot open a non-select statement" (this may be fixed after FPC 2.6)

To call a Stored Procedure or to issue other SQL commands that return no rowsets (CREATE,INSERT,UPDATE,DELETE), use query.ExecSQL instead of query.Open

Stored Procedure parameters work, e.g. 'EXEC MySP @param1=23'

The use of Parameters is recommended to avoid SQL injection vulnerabilities. In this case the SQL statement isn't built with values directly, but a named placeholder is used instead - and then the parameter is specified separately. Using this method it isn't possible to inject malicious SQL commands into variable fields in an attempt to tamper with the database; the server then knows specifically what part of the statement is a value and what is command. For example, instead of building a WHERE clause directly with the value 95 ('... WHERE field2>95'), instead use a named parameter like 'testval' -

 query.SQL.Text := 'select * from mytable where field2>:testval';
 query.Params.ParamByName('testval').AsString := '95';
 query.open;

Debugging ODBC & errors

Problems with transactions

In Lazarus 1.2.4 (FPC 2.6.4), ODBC transaction behaviour has changed. This may cause previously working code to stop working.

For a solution, see User_Changes_2.6.4#TODBCConnection_.28odbcconn.29_No_longer_autocommit

Error Messages

Each ODBC API call also returns a succes code; if there is an error, more information can be retrieved using calls to SQLGetDiagRec. (For the sake of completeness: diagnostic records are also available when an API call returns SQL_SUCCES_WITH_INFO.)

TODBCConnection checks the return code of each ODBC call and contructs an EODBCException. The message of this exception consists of:

  • a message identifying what the TODBCConnection was doing when the error occurred
  • the return code of the ODBC API call (e.g. SQL_ERROR)
  • a number of diagnostic records (obtained using SQLGetDiagRec); each record consists of three fields:
    • a 5-character error code identifying the error
    • a 'native error code'
    • a message describing the error

Function sequence error

If you get a 'Function sequence error' in the finalization section of the ODBCConn unit, then you probably did not properly clean up all you queries and connections.

Rollback does not work

If you issue
TSQLTransaction.Rollback
commands but your SQL is committed regardless, your ODBC connection probably is in AUTOCOMMIT mode; see the remarks about the AUTOCOMMIT connection parameter on how to correct this.

Tracing

Most ODBC managers have a tracing option in which all ODBC API calls are logged to a trace log. This can be very useful for debugging an ODBC application. The ODBC Data Source Administration GUI of both Windows and unixODBC have a tab where you can configure the tracing option.

Of course the trace log is mainly useful for developers that are familiar with the ODBC API, but it can help to identify the problem. Also, you can attach a trace log if you report a problem to the bug tracker.

See also

  • MS Access Using ODBC to access Microsoft Access databases

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