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 .
The table below lists the drivers included with Qt:
| 驱动程序名称 | DBMS |
|---|---|
| QDB2 | IBM DB2 (version 7.1 and above) |
| QIBASE | Borland InterBase |
| QMYSQL | MySQL |
| QOCI | Oracle Call Interface Driver |
| QODBC | Open Database Connectivity (ODBC) - Microsoft SQL Server and other ODBC-compliant databases |
| QPSQL | PostgreSQL (versions 7.3 and above) |
| QSQLITE2 | SQLite version 2 |
| QSQLITE | SQLite version 3 |
| QTDS |
Sybase Adaptive Server
注意： obsolete from Qt 4.7 |
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.
注意： To build a driver plugin you need to have the appropriate client library for your Database Management System (DBMS). This provides access to the API exposed by the DBMS, and is typically shipped with it. Most installation programs also allow you to install "development libraries", and these are what you need. These libraries are responsible for the low-level communication with the DBMS.
注意： When using Qt under Open Source terms but with a proprietary database, verify the client library's license compatibility with the LGPL.
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 sqlite2 sqlite tds [all auto] -sqlite .............. Select used sqlite3 [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 the
*_INCDIR=
,
*_LIBDIR=
，或
*_PREFIX=
command-line options. For example, if your MySQL files are installed in
/usr/local/mysql
(or in
C:\mysql
on Windows), then pass the following parameter to configure:
MYSQL_PREFIX=/usr/local/mysql
(或
MYSQL_PREFIX=C:\mysql
for Windows). The particulars for each driver are explained below.
Due to the practicalities of dealing with external dependencies, only the SQLite3 plugin is shipped with binary builds of Qt. 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. If the Qt build is configured with
-prefix
, it is necessary to install the plugins after building them, too. For example:
cd $QTDIR/qtbase/src/plugins/sqldrivers/mysql make install
MySQL 5 introduces 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"); q.next(); qDebug() << q.value(0) << q.value(1); // outputs "42" and "43"
注意：
@outval1
and
@outval2
are variables local to the current connection and will not be affected by queries sent from another host or connection.
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
而不是
libmysqlclient
. This can be done by adding
MYSQL_LIBS=-lmysqld
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.
You need the MySQL header files, as well as the shared library
libmysqlclient.so
. Depending on your Linux distribution, you may need to install a package which is usually called "mysql-devel".
Tell
qmake
where to find the MySQL header files and shared libraries (here it is assumed that MySQL is installed in
/usr/local
) and run
make
:
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- MYSQL_PREFIX=/usr/local make sub-mysql
You need to get the MySQL installation files. Run
SETUP.EXE
and choose "Custom Install". Install the "Libs & Include Files" Module. Build the plugin as follows (here it is assumed that MySQL is installed in
C:\MySQL
):
cd %QTDIR%\qtbase\src\plugins\sqldrivers qmake -- MYSQL_INCDIR=C:/MySQL/include "MYSQL_LIBDIR=C:/MYSQL/MySQL Server <version>/lib/opt" nmake sub-mysql
If you are not using a Microsoft compiler, replace
nmake
with
mingw32-make
in the line above.
The Qt OCI plugin supports Oracle 9i, 10g and higher. After connecting to the Oracle server, 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.
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.
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.
For Oracle 10g, all you need is the "Instant Client Package - Basic" and "Instant Client Package - SDK". For Oracle prior to 10g, you require the standard Oracle client and the SDK packages.
Oracle library files required to build the driver:
libclntsh.so
(all versions)
libwtc9.so
(only Oracle 9)
Tell
qmake
where to find the Oracle header files and shared libraries and run make:
For Oracle version 9:
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- "OCI_INCDIR=$ORACLE_HOME/rdbms/public" OCI_LIBDIR=$ORACLE_HOME/lib "OCI_LIBS=-lclntsh -lwtc9" make sub-oci
For Oracle version 10, we assume that you installed the RPM packages of the Instant Client Package SDK (you need to adjust the version number accordingly):
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- OCI_INCDIR=/usr/include/oracle/10.1.0.3/client OCI_LIBDIR=/usr/lib/oracle/10.1.0.3/client/lib make sub-oci
注意： If you are using the Oracle Instant Client package, you will need to set LD_LIBRARY_PATH when building the OCI SQL plugin, and when running an application that uses the OCI SQL plugin. You can avoid this requirement by setting RPATH, and listing all of the libraries to link to. Here is an example:
configure OCI_INCDIR=/usr/include/oracle/10.1.0.3/client OCI_LIBDIR=/usr/lib/oracle/10.1.0.3/client/lib -R /usr/lib/oracle/10.1.0.3/client/lib "OCI_LIBS=-lclntsh -lnnz10" make
If you wish to build the OCI plugin manually with this method, the procedure looks like this:
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- OCI_INCDIR=/usr/include/oracle/10.1.0.3/client OCI_LIBDIR=/usr/lib/oracle/10.1.0.3/client/lib "OCI_LIBS=-Wl,-rpath,/usr/lib/oracle/10.1.0.3/client/lib -lclntsh -lnnz10" make sub-oci
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
):
cd %QTDIR%\qtbase\src\plugins\sqldrivers qmake -- OCI_INCDIR=c:/oracle/oci/include OCI_LIBDIR=c:/oracle/oci/lib/msvc nmake sub-oci
If you are not using a Microsoft compiler, replace
nmake
with
mingw32-make
in the line above.
When you run your application, you will also need to add the
oci.dll
path to your
PATH
environment variable:
set PATH=%PATH%;c:\oracle\bin
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.
注意： You should use the native driver, if it is available, instead of the ODBC driver. ODBC support can be used as a fallback for compliant databases if no native driver is available.
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("QODBC3"); 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.
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 使用 QSqlQuery::setForwardOnly ().
// STORED_PROC uses the return statement or returns multiple result sets QSqlQuery query; query.setForwardOnly(true); query.exec("{call STORED_PROC}");
注意： The value returned by the stored procedure's return statement is discarded.
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.
Some driver managers and drivers do not support UNICODE. To use the QODBC plugin with such drivers, it has to be compiled with Q_ODBC_VERSION_2 defined.
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.
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
qmake
where to find the unixODBC header files and shared libraries (here it is assumed that unixODBC is installed in
/usr/local/unixODBC
) and run
make
:
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- ODBC_PREFIX=/usr/local/unixODBC make sub-odbc
The ODBC header and include files should already be installed in the right directories. You just have to build the plugin as follows:
cd %QTDIR%\qtbase\src\plugins\sqldrivers qmake nmake sub-odbc
If you are not using a Microsoft compiler, replace
nmake
with
mingw32-make
in the line above.
The QPSQL driver supports version 7.3 and higher of the PostgreSQL server.
For more information about PostgreSQL visit http://www.postgresql.org .
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.
Binary Large Objects are supported through the
BYTEA
field type in PostgreSQL server versions >= 7.1.
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
() 采用
true
will have no effect.
警告： If you build the QPSQL plugin with PostgreSQL version 9.2 or later, then you must distribute your application with libpq version 9.2 or later. Otherwise, loading the QPSQL plugin will fail with the following message:
QSqlDatabase: QPSQL driver not loaded QSqlDatabase: available drivers: QSQLITE QMYSQL QMYSQL3 QODBC QODBC3 QPSQL QPSQL7 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(db); query.setForwardOnly(true); query.exec("SELECT * FROM table"); while (query.next()) { // Handle changes in every iteration of the loop QVariant 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(db); query1.setForwardOnly(true); query1.exec("select * FROM table1"); while (query1.next()) { value = query1.value(0).toInt(); if (value == 1) { QSqlQuery query2(db); 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.
注意： Some methods of QSqlDatabase like tables(), primaryIndex() implicity execute SQL queries, so these also cannot be used while navigating the results of forward-only query.
注意： QPSQL will print the following warning if it detects a loss of query results:
QPSQLDriver::getResult: Query results lost - probably discarded on executing another SQL query.
You need the PostgreSQL client library and headers installed.
To make
qmake
find the PostgreSQL header files and shared libraries, run
qmake
the following way (assuming that the PostgreSQL client is installed in
/usr
):
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- PSQL_INCDIR=/usr/include/pgsql make sub-psql
Install the appropriate PostgreSQL developer libraries for your compiler. Assuming that PostgreSQL was installed in
C:\psql
, build the plugin as follows:
cd %QTDIR%\qtbase\src\plugins\sqldrivers qmake -- PSQL_INCDIR=C:/psql/include PSQL_LIBDIR=C:/psql/lib/ms nmake sub-psql
Users of MinGW may wish to consult the following online document: PostgreSQL MinGW/Native Windows .
注意： TDS is no longer used by MS Sql Server, and is superseded by ODBC . QTDS is obsolete from Qt 4.7.
It is not possible to set the port with QSqlDatabase::setPort () due to limitations in the Sybase client library. Refer to the Sybase documentation for information on how to set up a Sybase client configuration file to enable connections to databases on non-default ports.
Under Unix, two libraries are available which support the TDS protocol:
Regardless of which library you use, the shared object file
libsybdb.so
is needed. Set the
SYBASE
environment variable to point to the directory where you installed the client library and execute
qmake
:
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- TDS_PREFIX=$SYBASE make sub-tds
You can either use the DB-Library supplied by Microsoft or the Sybase Open Client ( https://support.sap.com ). Configure will try to find NTWDBLIB.LIB to build the plugin:
cd %QTDIR%\qtbase\src\plugins\sqldrivers qmake nmake sub-tds
By default, the Microsoft library is used on Windows. If you want to force the use of the Sybase Open Client, you must define
Q_USE_SYBASE
in
%QTDIR%\qtbase\src\plugins\sqldrivers\tds\qsql_tds.cpp
.
If you are not using a Microsoft compiler, replace
nmake
with
mingw32-make
in the line 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 ()).
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- DB2_PREFIX=$DB2DIR make sub-db2
The DB2 header and include files should already be installed in the right directories. You just have to build the plugin as follows:
cd %QTDIR%\qtbase\src\plugins\sqldrivers qmake -- "DB2_PREFIX=<DB2 home>/sqllib" nmake sub-db2
If you are not using a Microsoft compiler, replace
nmake
with
mingw32-make
in the line above.
Qt SQLite 2 plugin is offered for compatibility. Whenever possible, use the version 3 plugin instead. The build instructions for version 3 apply to version 2 as well.
Qt SQLite plugin makes it possible to access SQLite 数据库。 SQLite is an in-process database, which means that it is not necessary to have a database server. SQLite 对单个文件进行操作，必须将其设为数据库名称 (当打开连接时)。若文件不存在， SQLite 将试着创建它。 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
()).
在 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 .
SQLite
version 3 is included as a third-party library within Qt. It can be built by passing the
-qt-sqlite
parameter to the configure script.
If you do not want to use the
SQLite
library included with Qt, you can pass
-system-sqlite
to the configure script 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):
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- -system-sqlite SQLITE3_PREFIX=$SQLITE make sub-sqlite
在 Windows：
cd %QTDIR%\qtbase\src\plugins\sqldrivers qmake -- -system-sqlite SQLITE3_PREFIX=C:/SQLITE nmake sub-sqlite
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.
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
.
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.
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.
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"); db.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.
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 ()。范例：
QSqlQuery q; q.exec("execute procedure my_procedure"); q.next(); qDebug() << q.value(0); // outputs the first RETURN/OUT value
The following assumes InterBase or Firebird is installed in
/opt/interbase
:
If you are using InterBase:
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- IBASE_PREFIX=/opt/interbase make sub-ibase
If you are using Firebird, the Firebird library has to be set explicitly:
cd $QTDIR/qtbase/src/plugins/sqldrivers qmake -- IBASE_PREFIX=/opt/interbase IBASE_LIBS=-lfbclient make sub-ibase
The following assumes InterBase or Firebird is installed in
C:\interbase
:
If you are using InterBase:
cd %QTDIR%\qtbase\src\plugins\sqldrivers qmake -- IBASE_INCDIR=C:/interbase/include nmake sub-ibase
If you are using Firebird, the Firebird library has to be set explicitly:
cd %QTDIR%\qtbase\src\plugins\sqldrivers qmake -- IBASE_INCDIR=C:/interbase/include IBASE_LIBS=-lfbclient nmake sub-ibase
If you are not using a Microsoft compiler, replace
nmake
with
mingw32-make
in the line above.
注意，
C:\interbase\bin
must be in the
PATH
.
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 distibution 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:
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
运行
章节的
Project
panel to include the path to the folder containing the client libraries.
QT_DEBUG_COMPONENT
defined to get very verbose debug output when loading plugins.
Make sure you have followed the guide to 部署插件 .
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 () 和 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
，和
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
如何创建 Qt 插件
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() {} QSqlResult *createResult() const override { return new XyzResult(this); } };