SQL Database Drivers

The Qt SQL module uses driver plugins to communicate with the different database APIs. Since Qt's SQL Module API is database-independent, all database-specific code is contained within these drivers. Several drivers are supplied with Qt, and other drivers can be added. The driver source code is supplied and can be used as a model for writing your own drivers.

Supported Databases

The table below lists the drivers included with Qt:

SQLite is the in-process database system with the best test coverage and support on all platforms. Oracle via OCI, PostgreSQL, and MySQL through either ODBC or a native driver are well-tested on Windows and Linux. The completeness of the support for other systems depends on the availability and quality of client libraries.

Building the Drivers

Compile Qt with a specific driver

The Qt configure script tries to automatically detect the available client libraries on your machine. Run configure -help to see what drivers can be built. You should get an output similar to this:

[...]

Database options:

  -sql-<driver> ........ Enable SQL <driver> plugin. Supported drivers:
                         db2 ibase mysql oci odbc psql sqlite
                         [all auto]
  -sqlite .............. Select used sqlite [system/qt]

[...]

The configure script cannot detect the necessary libraries and include files if they are not in the standard paths, so it may be necessary to specify these paths using either driver-specific include and library path variables or CMAKE_INCLUDE_PATH and CMAKE_LIBRARY_PATH. For example, if your MySQL files are installed in C:\mysql-connector-c-6.1.11-winx64 on Windows, then pass the following parameter to double-dash part of configure line:

C:\Qt\6.0.0\Src\configure.bat -sql-mysql -- -DMySQL_INCLUDE_DIR="C:\mysql-8.0.22-winx64\include" -DMySQL_LIBRARY="C:\mysql-8.0.22-winx64\lib\libmysql.lib"
Configure summary:

...
Qt Sql Drivers:
  DB2 (IBM) .............................. no
  InterBase .............................. no
  MySql .................................. yes
  OCI (Oracle) ........................... no
  ODBC ................................... yes
  PostgreSQL ............................. no
  SQLite ................................. yes
    Using system provided SQLite ......... no
...

When you configure drivers in the manner described above, CMake skips any dependency checks and uses the provided paths as is. This is especially useful if the package provides its own set of system libraries that should not be recognized by the build routine.

In some cases it's more convenient to use CMAKE_INCLUDE_PATH and CMAKE_LIBRARY_PATH variables to locate required libraries. You should prefer this method if module needs to set properties for the provided target libraries (e.g. this is required for PostgreSQL and SQLite). For example, you can do this as follows, to locate MySQL:

C:\Qt\6.0.0\Src\configure.bat -sql-mysql -- -DCMAKE_INCLUDE_PATH="C:\mysql-8.0.22-winx64\include" -DCMAKE_LIBRARY_PATH="C:\mysql-8.0.22-winx64\lib"
Configure summary:

...
Qt Sql Drivers:
  DB2 (IBM) .............................. no
  InterBase .............................. no
  MySql .................................. yes
  OCI (Oracle) ........................... no
  ODBC ................................... yes
  PostgreSQL ............................. no
  SQLite ................................. yes
    Using system provided SQLite ......... no
...

The particulars for each driver are explained below.

Compile only a specific sql driver

A typical qt-cmake run (in this case to configure for MySQL) looks like this:

C:\Qt\6.0.0\mingw81_64\bin\qt-cmake -G Ninja C:\Qt\6.0.0\Src\qtbase\src\plugins\sqldrivers -DMySQL_INCLUDE_DIR="C:\mysql-8.0.22-winx64\include" -DMySQL_LIBRARY="C:\mysql-8.0.22-winx64\lib\libmysql.lib" -DCMAKE_INSTALL_PREFIX="C:\Qt\6.0.0\mingw81_64"
Configure summary:

Qt Sql Drivers:
  DB2 (IBM) .............................. no
  InterBase .............................. no
  MySql .................................. yes
  OCI (Oracle) ........................... no
  ODBC ................................... yes
  PostgreSQL ............................. no
  SQLite ................................. yes
    Using system provided SQLite ......... no

-- Configuring done
-- Generating done
-- Build files have been written to: C:/build-qt6-sqldrivers

Due to the practicalities of dealing with external dependencies, only the SQLite plugin is shipped with binary builds of Qt. Binary builds of Qt for Windows also include the ODBC plugin. To be able to add additional drivers to the Qt installation without re-building all of Qt, it is possible to configure and build the qtbase/src/plugins/sqldrivers directory outside of a full Qt build directory. Note that it is not possible to configure each driver separately, only all of them at once. Drivers can be built separately, though.

Driver Specifics

QMYSQL for MySQL or MariaDB 5.6 and higher

MariaDB is a fork of MySQL intended to remain free and open-source software under the GNU General Public License. MariaDB intended to maintain high compatibility with MySQL, ensuring a drop-in replacement capability with library binary parity and exact matching with MySQL APIs and commands. Therefore the plugin for MySQL and MariaDB are combined into one Qt plugin.

QMYSQL Stored Procedure Support

MySQL 5 has stored procedure support at the SQL level, but no API to control IN, OUT, and INOUT parameters. Therefore, parameters have to be set and read using SQL commands instead of QSqlQuery::bindValue().

Example stored procedure:

create procedure qtestproc (OUT param1 INT, OUT param2 INT)
BEGIN
    set param1 = 42;
    set param2 = 43;
END

Source code to access the OUT values:

QSqlQuery q;
q.exec("call qtestproc (@outval1, @outval2)");
q.exec("select @outval1, @outval2");
if (q.next())
    qDebug() << q.value(0) << q.value(1); // outputs "42" and "43"

Embedded MySQL Server

The MySQL embedded server is a drop-in replacement for the normal client library. With the embedded MySQL server, a MySQL server is not required to use MySQL functionality.

To use the embedded MySQL server, simply link the Qt plugin to libmysqld instead of libmysqlclient. This can be done by adding -DMySQL_LIBRARY=<path/to/mysqld/>libmysqld.<so|lib|dylib> to the configure command line.

Please refer to the MySQL documentation, chapter "libmysqld, the Embedded MySQL Server Library" for more information about the MySQL embedded server.

How to Build the QMYSQL Plugin on Unix and macOS

You need the MySQL / MariaDB header files, as well as the shared library libmysqlclient.<so|dylib> / libmariadb.<so|dylib>. Depending on your Linux distribution, you may need to install a package which is usually called "mysql-devel" or "mariadb-devel".

Tell qt-cmake where to find the MySQL / MariaDB header files and shared libraries (here it is assumed that MySQL / MariaDB is installed in /usr/local) and build:

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>/Src/qtbase/src/plugins/sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>/<platform> -DMySQL_INCLUDE_DIR="/usr/local/mysql/include" -DMySQL_LIBRARY="/usr/local/mysql/lib/libmysqlclient.<so|dylib>"
cmake --build .
cmake --install .

How to Build the QMYSQL Plugin on Windows

You need to get the MySQL installation files (e.g. mysql-installer-web-community-8.0.22.0.msi or mariadb-connector-c-3.1.11-win64.msi). Run the installer, select custom installation and install the MySQL C Connector which matches your Qt installation (x86 or x64). After installation check that the needed files are there:

• <MySQL dir>/lib/libmysql.lib
• <MySQL dir>/lib/libmysql.dll
• <MySQL dir>/include/mysql.h

and for MariaDB

• <MariaDB dir>/lib/libmariadb.lib
• <MariaDB dir>/lib/libmariadb.dll
• <MariaDB dir>/include/mysql.h

Build the plugin as follows (here it is assumed that <MySQL dir> is C:\mysql-8.0.22-winx64):

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>\Src\qtbase\src\plugins\sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>\<platform> -DMySQL_INCLUDE_DIR="C:\mysql-8.0.22-winx64\include" -DMySQL_LIBRARY="C:\mysql-8.0.22-winx64\lib\libmysql.lib"
cmake --build .
cmake --install .

When you distribute your application, remember to include libmysql.dll / libmariadb.dll in your installation package. It must be placed in the same folder as the application executable. libmysql.dll additionally needs the MSVC runtime libraries which can be installed with vcredist.exe

QOCI for the Oracle Call Interface (OCI)

The Qt OCI plugin supports connecting to Oracle database as determined by the version of the instant client used. This is dependent on what Oracle indicates it supports. The plugin will auto-detect the database version and enable features accordingly.

It's possible to connect to a Oracle database without a tnsnames.ora file. This requires that the database SID is passed to the driver as the database name, and that a hostname is given.

OCI User Authentication

The Qt OCI plugin supports authentication using external credentials (OCI_CRED_EXT). Usually, this means that the database server will use the user authentication provided by the operating system instead of its own authentication mechanism.

Leave the username and password empty when opening a connection with QSqlDatabase to use the external credentials authentication.

OCI BLOB/LOB Support

Binary Large Objects (BLOBs) can be read and written, but be aware that this process may require a lot of memory. You should use a forward only query to select LOB fields (see QSqlQuery::setForwardOnly()).

Inserting BLOBs should be done using either a prepared query where the BLOBs are bound to placeholders or QSqlTableModel, which uses a prepared query to do this internally.

How to Build the OCI Plugin on Unix and macOS

All you need is the " - Basic" and "Instant Client Package - SDK".

Oracle library files required to build the driver:

• libclntsh.<so|dylib> (all versions)

Tell qt-cmake where to find the Oracle header files and shared libraries and build.

We assume that you installed the RPM packages of the Instant Client Package SDK (you need to adjust the version number accordingly):

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>/Src/qtbase/src/plugins/sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>/<platform> -DOracle_INCLUDE_DIR="/usr/include/oracle/21/client64" -DOracle_LIBRARY="/usr/lib/oracle/21/client64/lib/libclntsh.<so|dylib>"
cmake --build .
cmake --install .

How to Build the OCI Plugin on Windows

Choosing the option "Programmer" in the Oracle Client Installer from the Oracle Client Installation CD is generally sufficient to build the plugin. For some versions of Oracle Client, you may also need to select the "Call Interface (OCI)" option if it is available.

Build the plugin as follows (here it is assumed that Oracle Client is installed in C:\oracle and SDK is installed in C:\oracle\sdk):

mkdir build-sqldrivers
cd build-sqldrivers
qt-cmake -G Ninja <qt_installation_path>\Src\qtbase\src\plugins\sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>\<platform> -DOracle_INCLUDE_DIR="C:\oracle\sdk\include" -DOracle_LIBRARY="C:\oracle\oci.lib"
cmake --build .
cmake --install .

When you run your application, you will also need to add the oci.lib path to your PATH environment variable:

set PATH=%PATH%;C:\oracle

QODBC for Open Database Connectivity (ODBC)

ODBC is a general interface that allows you to connect to multiple DBMSs using a common interface. The QODBC driver allows you to connect to an ODBC driver manager and access the available data sources. Note that you also need to install and configure ODBC drivers for the ODBC driver manager that is installed on your system. The QODBC plugin then allows you to use these data sources in your Qt applications.

On Windows, an ODBC driver manager should be installed by default. For Unix systems, there are some implementations which must be installed first. Note that every end user of your application is required to have an ODBC driver manager installed, otherwise the QODBC plugin will not work.

When connecting to an ODBC datasource, you should pass the name of the ODBC datasource to the QSqlDatabase::setDatabaseName() function, rather than the actual database name.

The QODBC Plugin needs an ODBC compliant driver manager version 2.0 or later. Some ODBC drivers claim to be version-2.0-compliant, but do not offer all the necessary functionality. The QODBC plugin therefore checks whether the data source can be used after a connection has been established, and refuses to work if the check fails. If you do not like this behavior, you can remove the #define ODBC_CHECK_DRIVER line from the file qsql_odbc.cpp. Do this at your own risk!

By default, Qt instructs the ODBC driver to behave as an ODBC 2.x driver. However, for some driver-manager/ODBC 3.x-driver combinations (e.g., unixODBC/MaxDB ODBC), telling the ODBC driver to behave as a 2.x driver can cause the driver plugin to have unexpected behavior. To avoid this problem, instruct the ODBC driver to behave as a 3.x driver by setting the connect option "SQL_ATTR_ODBC_VERSION=SQL_OV_ODBC3" before you open your database connection. Note that this will affect multiple aspects of ODBC driver behavior, e.g., the SQLSTATEs. Before setting this connect option, consult your ODBC documentation about behavior differences you can expect.

When using the SAP HANA database, the connection has to be established using the option "SCROLLABLERESULT=TRUE", as the HANA ODBC driver does not provide scrollable results by default, e.g.:

QSqlDatabase db = QSqlDatabase::addDatabase("QODBC");
QString connectString = QStringLiteral(
    "DRIVER=/path/to/installation/libodbcHDB.so;"
    "SERVERNODE=hostname:port;"
    "UID=USER;"
    "PWD=PASSWORD;"
    "SCROLLABLERESULT=true");
db.setDatabaseName(connectString);

If you experience very slow access of the ODBC datasource, make sure that ODBC call tracing is turned off in the ODBC datasource manager.

Some drivers do not support scrollable cursors. In that case, only queries in forwardOnly mode can be used successfully.

ODBC Stored Procedure Support

With Microsoft SQL Server the result set returned by a stored procedure that uses the return statement, or returns multiple result sets, will be accessible only if you set the query's forward only mode to forward using QSqlQuery::setForwardOnly().

// STORED_PROC uses the return statement or returns multiple result sets
QSqlQuery query;
query.setForwardOnly(true);
query.exec("{call STORED_PROC}");

ODBC Unicode Support

The QODBC Plugin will use the Unicode API if UNICODE is defined. On Windows NT based systems, this is the default. Note that the ODBC driver and the DBMS must also support Unicode.

For the Oracle 9 ODBC driver (Windows), it is necessary to check "SQL_WCHAR support" in the ODBC driver manager otherwise Oracle will convert all Unicode strings to local 8-bit.

How to Build the ODBC Plugin on Unix and macOS

It is recommended that you use unixODBC. You can find the latest version and ODBC drivers at http://www.unixodbc.org. You need the unixODBC header files and shared libraries.

Tell qt-cmake where to find the unixODBC header files and shared libraries (here it is assumed that unixODBC is installed in /usr/local/unixODBC) and build:

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>/Src/qtbase/src/plugins/sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>/<platform> -DODBC_INCLUDE_DIR="/usr/local/unixODBC/include" -DODBC_LIBRARY="/usr/local/unixODBC/lib/libodbc.<so|dylib>"
cmake --build .
cmake --install .

How to Build the ODBC Plugin on Windows

The ODBC header and include files should already be installed in the right directories. You just have to build the plugin as follows:

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>\Src\qtbase\src\plugins\sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>\<platform>
cmake --build .
cmake --install .

QPSQL for PostgreSQL (Version 7.3 and Above)

The QPSQL driver supports version 7.3 and higher of the PostgreSQL server.

For more information about PostgreSQL visit http://www.postgresql.org.

QPSQL Unicode Support

The QPSQL driver automatically detects whether the PostgreSQL database you are connecting to supports Unicode or not. Unicode is automatically used if the server supports it. Note that the driver only supports the UTF-8 encoding. If your database uses any other encoding, the server must be compiled with Unicode conversion support.

Unicode support was introduced in PostgreSQL version 7.1 and it will only work if both the server and the client library have been compiled with multibyte support. More information about how to set up a multibyte enabled PostgreSQL server can be found in the PostgreSQL Administrator Guide, Chapter 5.

QPSQL Case Sensitivity

PostgreSQL databases will only respect case sensitivity if the table or field name is quoted when the table is created. So for example, a SQL query such as:

CREATE TABLE "testTable" ("id" INTEGER);

will ensure that it can be accessed with the same case that was used. If the table or field name is not quoted when created, the actual table name or field name will be lower-case. When QSqlDatabase::record() or QSqlDatabase::primaryIndex() access a table or field that was unquoted when created, the name passed to the function must be lower-case to ensure it is found. For example:

QString tableString("testTable");
QSqlQuery q;
// Create table query is not quoted, therefore it is mapped to lower case
q.exec(QString("CREATE TABLE %1 (id INTEGER)").arg(tableString));
// Call toLower() on the string so that it can be matched
QSqlRecord rec = database.record(tableString.toLower());

QPSQL Forward-only query support

To use forward-only queries, you must build the QPSQL plugin with PostreSQL client library version 9.2 or later. If the plugin is built with an older version, then forward-only mode will not be available - calling QSqlQuery::setForwardOnly() with true will have no effect.

QSqlDatabase: QPSQL driver not loaded
QSqlDatabase: available drivers: QSQLITE QMYSQL QMARIADB QODBC QPSQL
Could not create database object

While navigating the results in forward-only mode, the handle of QSqlResult may change. Applications that use the low-level handle of SQL result must get a new handle after each call to any of QSqlResult fetch functions. Example:

QSqlQuery query;
QVariant v;
query.setForwardOnly(true);
query.exec("SELECT * FROM table");
while (query.next()) {
    // Handle changes in every iteration of the loop
    v = query.result()->handle();

    if (qstrcmp(v.typeName(), "PGresult*") == 0) {
        PGresult *handle = *static_cast<PGresult **>(v.data());
        if (handle) {
            // Do something...
        }
    }
}

While reading the results of a forward-only query with PostgreSQL, the database connection cannot be used to execute other queries. This is a limitation of libpq library. Example:

int value;
QSqlQuery query1;
query1.setForwardOnly(true);
query1.exec("select * FROM table1");
while (query1.next()) {
    value = query1.value(0).toInt();
    if (value == 1) {
        QSqlQuery query2;
        query2.exec("update table2 set col=2");  // WRONG: This will discard all results of
    }                                            // query1, and cause the loop to quit
}

This problem will not occur if query1 and query2 use different database connections, or if we execute query2 after the while loop.

QPSQLDriver::getResult: Query results lost - probably discarded on executing another SQL query.

How to Build the QPSQL Plugin on Unix and macOS

You need the PostgreSQL client library and headers installed.

To make qt-cmake find the PostgreSQL header files and shared libraries, build the plugin the following way (assuming that the PostgreSQL client is installed in /usr/local/pgsql):

mkdir build-psql-driver
cd build-psql-driver

qt-cmake -G Ninja <qt_installation_path>/Src/qtbase/src/plugins/sqldrivers-DCMAKE_INSTALL_PREFIX=<qt_installation_path>/<platform> -DCMAKE_INCLUDE_PATH="/usr/local/pgsql/include" -DCMAKE_LIBRARY_PATH="/usr/local/pgsql/lib"
cmake --build .
cmake --install .

How to Build the QPSQL Plugin on Windows

Install the appropriate PostgreSQL developer libraries for your compiler. Assuming that PostgreSQL was installed in C:\pgsql, build the plugin as follows:

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>\Src\qtbase\src\plugins\sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>\<platform> -DCMAKE_INCLUDE_PATH="C:\pgsql\include" -DCMAKE_LIBRARY_PATH="C:\pgsql\lib"
cmake --build .
cmake --install .

Users of MinGW may wish to consult the following online document: PostgreSQL MinGW/Native Windows.

When you distribute your application, remember to include libpq.dll in your installation package. It must be placed in the same folder as the application executable.

QDB2 for IBM DB2 (Version 7.1 and Above)

The Qt DB2 plugin makes it possible to access IBM DB2 databases. It has been tested with IBM DB2 v7.1 and 7.2. You must install the IBM DB2 development client library, which contains the header and library files necessary for compiling the QDB2 plugin.

The QDB2 driver supports prepared queries, reading/writing of Unicode strings and reading/writing of BLOBs.

We suggest using a forward-only query when calling stored procedures in DB2 (see QSqlQuery::setForwardOnly()).

How to Build the QDB2 Plugin on Unix and macOS

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>/Src/qtbase/src/plugins/sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>/<platform> -DDB2_INCLUDE_DIR="/usr/local/db2/include" -DDB2_LIBRARY="/usr/local/db2/lib/libdb2.<so|dylib>"
cmake --build .
cmake --install .

How to Build the QDB2 Plugin on Windows

The DB2 header and include files should already be installed in the right directories. You just have to build the plugin as follows:

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>\Src\qtbase\src\plugins\sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>\<platform> -DDB2_INCLUDE_DIR="C:\db2\include" -DDB2_LIBRARY="C:\db2\lib\db2.lib"
cmake --build .
cmake --install .

QSQLITE for SQLite (Version 3 and Above)

The Qt SQLite plugin makes it possible to access SQLite databases. SQLite is an in-process database, which means that it is not necessary to have a database server. SQLite operates on a single file, which must be set as the database name when opening a connection. If the file does not exist, SQLite will try to create it. SQLite also supports in-memory and temporary databases. Simply pass respectively ":memory:" or an empty string as the database name.

SQLite has some restrictions regarding multiple users and multiple transactions. If you try to read/write on a resource from different transactions, your application might freeze until one transaction commits or rolls back. The Qt SQLite driver will retry to write to a locked resource until it runs into a timeout (see QSQLITE_BUSY_TIMEOUT at QSqlDatabase::setConnectOptions()).

In SQLite any column, with the exception of an INTEGER PRIMARY KEY column, may be used to store any type of value. For instance, a column declared as INTEGER may contain an integer value in one row and a text value in the next. This is due to SQLite associating the type of a value with the value itself rather than with the column it is stored in. A consequence of this is that the type returned by QSqlField::type() only indicates the field's recommended type. No assumption of the actual type should be made from this and the type of the individual values should be checked.

The driver is locked for updates while a select is executed. This may cause problems when using QSqlTableModel because Qt's item views fetch data as needed (with QSqlQuery::fetchMore() in the case of QSqlTableModel).

You can find information about SQLite on http://www.sqlite.org.

How to Build the QSQLITE Plugin

SQLite version 3 is included as a third-party library within Qt. It can be built by passing the -DFEATURE_system_sqlite=OFF parameter to the qt-cmake command line.

If you do not want to use the SQLite library included with Qt, you can pass -DFEATURE_system_sqlite=ON to the qt-cmake command line to use the SQLite libraries of the operating system. This is recommended whenever possible, as it reduces the installation size and removes one component for which you need to track security advisories.

On Unix and macOS (replace $SQLITE with the directory where SQLite resides):

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>/Src/qtbase/src/plugins/sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>/<platform> -DFEATURE_system_sqlite=ON -DCMAKE_INCLUDE_PATH="$SQLITE/include" -DCMAKE_LIBRARY_PATH="$SQLITE/lib"
cmake --build .
cmake --install .

On Windows (assuming that SQLite is installed in C:\SQLITE):

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>\Src\qtbase\src\plugins\sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>\<platform> -DFEATURE_system_sqlite=ON -DCMAKE_INCLUDE_PATH="C:\SQLITE\include" -DCMAKE_LIBRARY_PATH="C:\SQLITE\lib"
cmake --build .
cmake --install .

Enable REGEXP operator

SQLite comes with a REGEXP operation. However the needed implementation must be provided by the user. For convenience a default implementation can be enabled by setting the connect option QSQLITE_ENABLE_REGEXP before the database connection is opened. Then a SQL statement like "column REGEXP 'pattern'" basically expands to the Qt code

column.contains(QRegularExpression("pattern"));

For better performance the regular expressions are cached internally. By default the cache size is 25, but it can be changed through the option's value. For example passing "QSQLITE_ENABLE_REGEXP=10" reduces the cache size to 10.

QSQLITE File Format Compatibility

SQLite minor releases sometimes break file format forward compatibility. For example, SQLite 3.3 can read database files created with SQLite 3.2, but databases created with SQLite 3.3 cannot be read by SQLite 3.2. Please refer to the SQLite documentation and change logs for information about file format compatibility between versions.

Qt minor releases usually follow the SQLite minor releases, while Qt patch releases follow SQLite patch releases. Patch releases are therefore both backward and forward compatible.

To force SQLite to use a specific file format, it is necessary to build and ship your own database plugin with your own SQLite library as illustrated above. Some versions of SQLite can be forced to write a specific file format by setting the SQLITE_DEFAULT_FILE_FORMAT define when building SQLite.

QIBASE for Borland InterBase

The Qt InterBase plugin makes it possible to access the InterBase and Firebird databases. InterBase can either be used as a client/server or without a server in which case it operates on local files. The database file must exist before a connection can be established. Firebird must be used with a server configuration.

Note that InterBase requires you to specify the full path to the database file, no matter whether it is stored locally or on another server.

QSqlDatabase db;
db.setHostName("MyServer");
db.setDatabaseName("C:\\test.gdb");

You need the InterBase/Firebird development headers and libraries to build this plugin.

Due to license incompatibilities with the GPL, users of the Qt Open Source Edition are not allowed to link this plugin to the commercial editions of InterBase. Please use Firebird or the free edition of InterBase.

QIBASE Unicode Support and Text Encoding

By default the driver connects to the database using UNICODE_FSS. This can be overridden by setting the ISC_DPB_LC_CTYPE parameter with QSqlDatabase::setConnectOptions() before opening the connection.

// connect to database using the Latin-1 character set
db.setConnectOptions("ISC_DPB_LC_CTYPE=Latin1");
if (db.open())
    qDebug("The database connection is open.");

If Qt does not support the given text encoding the driver will issue a warning message and connect to the database using UNICODE_FSS.

Note that if the text encoding set when connecting to the database is not the same as in the database, problems with transliteration might arise.

QIBASE Stored procedures

InterBase/Firebird return OUT values as result set, so when calling stored procedure, only IN values need to be bound via QSqlQuery::bindValue(). The RETURN/OUT values can be retrieved via QSqlQuery::value(). Example:

QSqlQuery q;
q.exec("execute procedure my_procedure");
if (q.next())
    qDebug() << q.value(0); // outputs the first RETURN/OUT value

How to Build the QIBASE Plugin on Unix and macOS

The following assumes InterBase or Firebird is installed in /opt/interbase:

If you are using InterBase:

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>/Src/qtbase/src/plugins/sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>/<platform> -DInterbase_INCLUDE_DIR="/opt/interbase/include" -DInterbase_LIBRARY="/opt/interbase/lib/libgds.<so|dylib>"
cmake --build .
cmake --install .

If you are using Firebird, the Firebird library has to be set explicitly:

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>/Src/qtbase/src/plugins/sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>/<platform> -DInterbase_INCLUDE_DIR="/opt/interbase/include" -DInterbase_LIBRARY="/opt/interbase/lib/libfbclient.<so|dylib>"
cmake --build .
cmake --install .

How to Build the QIBASE Plugin on Windows

The following assumes InterBase or Firebird is installed in C:\interbase:

If you are using InterBase:

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>\Src\qtbase\src\plugins\sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>\<platform> -DInterbase_INCLUDE_DIR="C:\interbase\include" -DInterbase_LIBRARY="C:\interbase\gds.lib"
cmake --build .
cmake --install .

If you are using Firebird:

mkdir build-sqldrivers
cd build-sqldrivers

qt-cmake -G Ninja <qt_installation_path>\Src\qtbase\src\plugins\sqldrivers -DCMAKE_INSTALL_PREFIX=<qt_installation_path>\<platform> -DInterbase_INCLUDE_DIR="C:\interbase\include" -DInterbase_LIBRARY="C:\interbase\lib\fbclient_ms.lib"
cmake --build .
cmake --install .

Note that C:\interbase\bin must be in the PATH.

Troubleshooting

You should always use client libraries that have been compiled with the same compiler as you are using for your project. If you cannot get a source distribution to compile the client libraries yourself, you must make sure that the pre-compiled library is compatible with your compiler, otherwise you will get a lot of "undefined symbols" errors. Some compilers have tools to convert libraries, e.g. Borland ships the tool COFF2OMF.EXE to convert libraries that have been generated with Microsoft Visual C++.

If the compilation of a plugin succeeds but it cannot be loaded, make sure that the following requirements are met:

• Ensure that the plugin is in the correct directory. You can use QApplication::libraryPaths() to determine where Qt looks for plugins.
• Ensure that the client libraries of the DBMS are available on the system. On Unix, run the command ldd and pass the name of the plugin as parameter, for example ldd libqsqlmysql.so. You will get a warning if any of the client libraries could not be found. On Windows, you can use Visual Studio's dependency walker. With Qt Creator, you can update the PATH environment variable in the Run section of the Project panel to include the path to the folder containing the client libraries.
• Compile Qt with QT_DEBUG_COMPONENT defined to get very verbose debug output when loading plugins.

Make sure you have followed the guide to Deploying Plugins.

How to Write Your Own Database Driver

QSqlDatabase is responsible for loading and managing database driver plugins. When a database is added (see QSqlDatabase::addDatabase()), the appropriate driver plugin is loaded (using QSqlDriverPlugin). QSqlDatabase relies on the driver plugin to provide interfaces for QSqlDriver and QSqlResult.

QSqlDriver is an abstract base class which defines the functionality of a SQL database driver. This includes functions such as QSqlDriver::open() and QSqlDriver::close(). QSqlDriver is responsible for connecting to a database, establish the proper environment, etc. In addition, QSqlDriver can create QSqlQuery objects appropriate for the particular database API. QSqlDatabase forwards many of its function calls directly to QSqlDriver which provides the concrete implementation.

QSqlResult is an abstract base class which defines the functionality of a SQL database query. This includes statements such as SELECT, UPDATE, and ALTER TABLE. QSqlResult contains functions such as QSqlResult::next() and QSqlResult::value(). QSqlResult is responsible for sending queries to the database, returning result data, etc. QSqlQuery forwards many of its function calls directly to QSqlResult which provides the concrete implementation.

QSqlDriver and QSqlResult are closely connected. When implementing a Qt SQL driver, both of these classes must to be subclassed and the abstract virtual methods in each class must be implemented.

To implement a Qt SQL driver as a plugin (so that it is recognized and loaded by the Qt library at runtime), the driver must use the Q_PLUGIN_METADATA() macro. Read How to Create Qt Plugins for more information on this. You can also check out how this is done in the SQL plugins that are provided with Qt in QTDIR/qtbase/src/plugins/sqldrivers.

The following code can be used as a skeleton for a SQL driver:

class XyzResult : public QSqlResult
{
public:
    XyzResult(const QSqlDriver *driver)
        : QSqlResult(driver) {}
    ~XyzResult() {}

protected:
    QVariant data(int /* index */) override { return QVariant(); }
    bool isNull(int /* index */) override { return false; }
    bool reset(const QString & /* query */) override { return false; }
    bool fetch(int /* index */) override { return false; }
    bool fetchFirst() override { return false; }
    bool fetchLast() override { return false; }
    int size() override { return 0; }
    int numRowsAffected() override { return 0; }
    QSqlRecord record() const override { return QSqlRecord(); }
};

class XyzDriver : public QSqlDriver
{
public:
    XyzDriver() {}
    ~XyzDriver() {}

    bool hasFeature(DriverFeature /* feature */) const override { return false; }
    bool open(const QString & /* db */, const QString & /* user */,
              const QString & /* password */, const QString & /* host */,
              int /* port */, const QString & /* options */) override
        { return false; }
    void close() override {}
    QSqlResult *createResult() const override { return new XyzResult(this); }
};