// Copyright (C) 2020 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \page sql-driver.html
    \title SQL Database Drivers
    \brief How to configure and install Qt SQL drivers for supported databases.

    The Qt SQL module uses driver \l{How to Create Qt
    Plugins}{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
    \l{#development}{writing your own drivers}.

    \section1 Supported Databases

    The table below lists the drivers included with Qt:

    \table
    \header \li Driver name \li DBMS
    \row \li \l{#QDB2}{QDB2} \li IBM DB2 (version 7.1 and above)
    \row \li \l{#QIBASE}{QIBASE} \li Borland InterBase / Firebird
    \row \li \l{#QMYSQL}{QMYSQL / MARIADB} \li MySQL or MariaDB (version 5.6 and above)
    \row \li \l{#QOCI}{QOCI} \li Oracle Call Interface Driver (version 12.1 and above)
    \row \li \l{#QODBC}{QODBC}
         \li Open Database Connectivity (ODBC) - Microsoft SQL Server and other
            ODBC-compliant databases
    \row \li \l{#QPSQL}{QPSQL} \li PostgreSQL (versions 7.3 and above)
    \row \li \l{#QSQLITE}{QSQLITE} \li SQLite version 3
    \row \li \l{#QMIMER}{QMIMER} \li Mimer SQL (version 11 and above)
    \endtable

    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.

    \note 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. Also make sure to install
    the correct database libraries for your Qt architecture (32 or 64 bit).

    \note When using Qt under Open Source terms but with a proprietary
    database, verify the client library's license compatibility with
    the LGPL.

    \target building
    \section1 Building the Drivers
    \target DriverWithQt
    \section2 Compile Qt with a specific driver

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

    \snippet code/doc_src_sql-driver.qdoc 0

    The \c 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 \c CMAKE_INCLUDE_PATH
    and \c CMAKE_LIBRARY_PATH. For example, if your MySQL files are
    installed in \c{C:\mysql-connector-c-6.1.11-winx64} on Windows, then pass
    the following parameter to double-dash part of configure line:
    \snippet code/doc_src_sql-driver.qdoc 42
    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.

    The particulars for each driver are explained below.

    \note If something goes wrong and you want CMake to recheck your
    available drivers, you might need to remove \e{CMakeCache.txt} from the build
    directory.

    \target DriverStandalone
    \section2 Compile only a specific SQL driver

    It's possible to only compile a specific SQL driver when Qt is already built
    or installed as binary version. But you have to make sure to install
    the exact same version of the Qt sources (for example through the \QMT) -
    otherwise you might get compile errors due to changed apis.
    Also make sure to properly set up the build environment by executing the
    appropriate Qt command prompt in the Windows Start menu.

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

    \snippet code/doc_src_sql-driver.qdoc 41

    After configuring with \c qt-cmake, build the driver by running \c ninja.

    \note As mentioned in \l{#DriverWithQt}{Compile Qt with a specific driver},
    if the driver could not be found or is not enabled, start over by removing
    \e{CMakeCache.txt}.

    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 and PostgreSQL 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 \c qtbase/src/plugins/sqldrivers directory outside
    of a full Qt build directory. Note that it is not possible to
    \e configure each driver separately, only all of them at once.
    Drivers can be \e built separately, though.

    \note You need to specify \c{CMAKE_INSTALL_PREFIX}, if you want to install
    plugins after the build is finished.

    \section1 Driver Specifics

    \target QMYSQL
    \section2 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.

    \section3 Timestamp Support
    Since Qt 6.8, QDateTime values are converted to UTC before insertion and
    back from UTC during retrieval. To make this work, the driver sets
    the connection time zone to UTC during open() (SET time_zone = '+00:00').
    Since MySQL does not store any timezone information, this information is
    lost and all retrieved QDateTime values are UTC.

    \section3 QMYSQL Stored Procedure Support

    MySQL 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:

    \snippet code/doc_src_sql-driver.qdoc 1

    Source code to access the OUT values:

    \snippet code/doc_src_sql-driver.cpp 2

    \note \c{@outval1} and \c{@outval2} are variables local to the current
    connection and will not be affected by queries sent from another host
    or connection.

    \section3 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 \c
    libmysqld instead of \c libmysqlclient. This can be done by adding
    \c{-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.

    \section3 Connection options
    The Qt MySQL/MariaDB plugin honors the following connection options:
    \table
    \header \li Attribute \li Possible value
    \row
      \li CLIENT_COMPRESS
      \li If set, switches to compressed protocol after successful authentication
    \row
      \li CLIENT_FOUND_ROWS
      \li If set, send found rows instead of affected rows
    \row
      \li CLIENT_IGNORE_SPACE
      \li If set, ignore spaces before '('
    \row
      \li CLIENT_NO_SCHEMA
      \li If set, don't allow database.table.column
    \row
      \li CLIENT_INTERACTIVE
      \li If set, client is treated as interactive
    \row
      \li MYSQL_OPT_PROTOCOL
      \li explicitly specify the protocol to use:\br
          MYSQL_PROTOCOL_TCP: use tcp connection (ip/hostname specified through setHostname())
          MYSQL_PROTOCOL_SOCKET: connect through a socket specified in UNIX_SOCKET
          MYSQL_PROTOCOL_PIPE: connect through a named pipe specified in UNIX_SOCKET
          MYSQL_PROTOCOL_MEMORY: connect through shared memory specified in MYSQL_SHARED_MEMORY_BASE_NAME
    \row
      \li UNIX_SOCKET
      \li Specifies the socket or named pipe to use, even it's called UNIX_SOCKET it
          can also be used on windows
    \row
      \li MYSQL_SHARED_MEMORY_BASE_NAME
      \li Specified the shared memory segment name to use
    \row
      \li MYSQL_OPT_RECONNECT
      \li TRUE or 1: Automatically reconnect after connection loss\br
          FALSE or 0: No automatic reconnect after connection loss (default)\br
          See \l {https://dev.mysql.com/doc/c-api/8.0/en/c-api-auto-reconnect.html}
          {Automatic Reconnection Control}
    \row
      \li MYSQL_OPT_CONNECT_TIMEOUT
      \li The connect timeout in seconds
    \row
      \li MYSQL_OPT_READ_TIMEOUT
      \li The timeout in seconds for each attempt to read from the server
    \row
      \li MYSQL_OPT_WRITE_TIMEOUT
      \li The timeout in seconds for each attempt to write to the server
    \row
      \li MYSQL_OPT_LOCAL_INFILE
      \li Set to 1 to enable the support for local
          \l {https://dev.mysql.com/doc/refman/8.0/en/load-data.html} {LOAD_DATA},
          disabled if not set or 0
    \row
      \li MYSQL_OPT_SSL_MODE
      \li The security state to use for the connection to the server: SSL_MODE_DISABLED,
          SSL_MODE_PREFERRED, SSL_MODE_REQUIRED, SSL_MODE_VERIFY_CA, SSL_MODE_VERIFY_IDENTITY.
          Only available when linked against MySQL 5.7.10 or higher.
    \row
      \li MYSQL_OPT_TLS_VERSION
      \li A list of protocols the client permits for encrypted connections. The value can be
          a combination of 'TLSv1' ,' TLSv1.1', 'TLSv1.2' or 'TLSv1.3' depending on the used \l
          {https://dev.mysql.com/doc/refman/8.0/en/encrypted-connection-protocols-ciphers.html#encrypted-connection-protocol-configuration}
          {MySQL server} version.
          Only available when linked against MySQL 5.7.11 or higher or MariaDB C Connector 3.1.10.
    \row
      \li MYSQL_OPT_SSL_KEY / SSL_KEY (deprecated)
      \li The path name of the client private key file
    \row
      \li MYSQL_OPT_SSL_CERT / SSL_CERT (deprecated)
      \li The path name of the client public key certificate file
    \row
      \li MYSQL_OPT_SSL_CA / SSL_CA (deprecated)
      \li The path name of the Certificate Authority (CA) certificate file
    \row
      \li MYSQL_OPT_SSL_CAPATH / SSL_CAPATH (deprecated)
      \li The path name of the directory that contains trusted SSL CA certificate files
    \row
      \li MYSQL_OPT_SSL_CIPHER / SSL_CIPHER (deprecated)
      \li The list of permissible ciphers for SSL encryption
    \row
      \li MYSQL_OPT_SSL_CRL
      \li The path name of the file containing certificate revocation lists
    \row
      \li MYSQL_OPT_SSL_CRLPATH
      \li The path name of the directory that contains files containing certificate revocation lists
    \row
      \li MYSQL_OPT_SSL_VERIFY_SERVER_CERT
      \li TRUE or 1: Enable verification of the server's Common Name identity (default)\br
          FALSE or 0: Enable verification of the server's Common Name identity\br
          Only available when linked against MySQL 5.7.11 or MariaDB, removed with MySQL 8.0.
    \endtable
    For more detailed information about the connect options please refer
    to the \l {https://dev.mysql.com/doc/c-api/8.0/en/mysql-options.html}
    {mysql_options()} MySQL documentation.


    \section3 How to Build the QMYSQL Plugin on Unix and \macos

    You need the MySQL / MariaDB header files, as well as the shared library
    \c{libmysqlclient.<so|dylib>} / \c{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 \c qt-cmake where to find the MySQL / MariaDB header files and shared
    libraries (here it is assumed that MySQL / MariaDB is installed in
    \c{/usr/local}) and build:

    \snippet code/doc_src_sql-driver.qdoc 3

    \section3 How to Build the QMYSQL Plugin on Windows

    You need to get the MySQL installation files (e.g.
    \l {https://dev.mysql.com/downloads/installer/}{MySQL web installer} or
    \l {https://downloads.mariadb.com/Connectors/c/}{MariaDB C Connector}).
    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:
    \list
    \li \c {<MySQL dir>/lib/libmysql.lib}
    \li \c {<MySQL dir>/lib/libmysql.dll}
    \li \c {<MySQL dir>/include/mysql.h}
    \endlist
    and for MariaDB
    \list
    \li \c {<MariaDB dir>/lib/libmariadb.lib}
    \li \c {<MariaDB dir>/lib/libmariadb.dll}
    \li \c {<MariaDB dir>/include/mysql.h}
    \endlist

    \note As of MySQL 8.0.19, the C Connector is no longer offered as a standalone
    installable component. Instead, you can get \c{mysql.h} and \c{libmysql.*} by
    installing the full MySQL Server (x64 only) or the
    \l{https://downloads.mariadb.org/connector-c/}{MariaDB C Connector}.

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

    \snippet code/doc_src_sql-driver.qdoc 5

    When you distribute your application, remember to include \e libmysql.dll / \e libmariadb.dll
    in your installation package. It must be placed in the same folder
    as the application executable. \e libmysql.dll additionally needs the
    MSVC runtime libraries which can be installed with
    \l {https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads}{vcredist.exe}

    \target QOCI
    \section2 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.

    \section3 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.

    \section3 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.

    \section3 Connection options
    The Qt OCI plugin honors the following connection options:
    \table
    \header \li Attribute \li Possible value
    \row
      \li OCI_ATTR_PREFETCH_ROWS
      \li Sets the OCI attribute
          \l {https://docs.oracle.com/database/121/LNOCI/oci04sql.htm#LNOCI16355}
          {OCI_ATTR_PREFETCH_ROWS} to the specified value
    \row
      \li OCI_ATTR_PREFETCH_MEMORY
      \li Sets the OCI attribute
          \l {https://docs.oracle.com/database/121/LNOCI/oci04sql.htm#LNOCI16355}
          {OCI_ATTR_PREFETCH_MEMORY} to the specified value
    \row
      \li OCI_AUTH_MODE
      \li OCI_SYSDBA: authenticate for SYSDBA access\br
          OCI_SYSOPER: authenticate for SYSOPER access\br
          OCI_DEFAULT: authenticate with normal access\br
          see \l {https://docs.oracle.com/cd/B10500_01/appdev.920/a96584/oci15r13.htm}
          {OCISessionBegin} for more information about the access modes
    \endtable

    \section3 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:

    \list
    \li \c libclntsh.<so|dylib> (all versions)
    \endlist

    Tell \c 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):
    \snippet code/doc_src_sql-driver.qdoc 7

    \note 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.

    \section3 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{C:\oracle} and SDK is installed in \c{C:\oracle\sdk}):

    \snippet code/doc_src_sql-driver.qdoc 8

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

    \snippet code/doc_src_sql-driver.qdoc 9

    \target QODBC
    \section2 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.

    \note 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 is 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 (DSN) to the QSqlDatabase::setDatabaseName()
    function, rather than the actual database name.
    It's also possible to pass a FILEDSN (*.dsn) filename or a complete
    ODBC driver string. When passing a driver string you must make sure,
    that all parameters (username, password, ...) are properly escaped.
    Passing the username or password through the QSqlDatabase functions,
    the escaping is done by the QODBC plugin.

    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 \c{#define
    ODBC_CHECK_DRIVER} line from the file \c{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 \e{driver-manager/ODBC 3.x-driver}
    combinations (e.g., \e{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
    \l{QSqlDatabase::setConnectOptions()} {setting the connect option}
    \c{"SQL_ATTR_ODBC_VERSION=SQL_OV_ODBC3"} before you
    \l{QSqlDatabase::open()} {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.

    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 \l QSqlQuery::setForwardOnly() mode can be used successfully.

    \section3 Timestamp Support
    ODBC is using TIMESTAMP_STRUCT which has no information about any
    timezone or similar. Due to this, the QDateTime is used without honoring
    the timezone at all.
    \note: This might change in the future.

    \section3 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 \e forward using \l QSqlQuery::setForwardOnly().

    \snippet code/doc_src_sql-driver.cpp 10

    \note The value returned by the stored procedure's return statement
    is discarded.

    \section3 ODBC Unicode Support

    The QODBC Plugin will use the Unicode API if UNICODE is defined. On
    Windows 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 representation.

    \section3 Connection options
    The Qt ODBC plugin honors the following connection options:
    \table
    \header \li Attribute \li Possible value
    \row
      \li SQL_ATTR_ACCESS_MODE
      \li SQL_MODE_READ_ONLY: open the database in read-only mode\br
          SQL_MODE_READ_WRITE: open the database in read-write mode (default)
    \row
      \li SQL_ATTR_LOGIN_TIMEOUT
      \li Number of seconds to wait for the database connection
          during login (a value of 0 will wait forever)
    \row
      \li SQL_ATTR_CONNECTION_TIMEOUT
      \li Number of seconds to wait for any request to the database
          (a value of 0 will wait forever)
    \row
      \li SQL_ATTR_CURRENT_CATALOG
      \li The catalog (database) to use for this connection
    \row
      \li SQL_ATTR_METADATA_ID
      \li SQL_TRUE: the string argument of catalog functions are treated
          as identifiers\br
          SQL_FALSE: the string arguments of catalog functions are not
          treated as identifiers
    \row
      \li SQL_ATTR_PACKET_SIZE
      \li Specifies the network packet size in bytes
    \row
      \li SQL_ATTR_TRACEFILE
      \li A string containing the name of the trace file
    \row
      \li SQL_ATTR_TRACE
      \li SQL_OPT_TRACE_ON: Enable database query tracing\br
          SQL_OPT_TRACE_OFF: Disable database query tracing (default)
    \row
      \li SQL_ATTR_CONNECTION_POOLING
      \li Enable or disable connection pooling at the environment level.\br
          SQL_CP_DEFAULT, SQL_CP_OFF: Connection pooling is turned off (default)\br
          SQL_CP_ONE_PER_DRIVER: A single connection pool is supported for each driver\br
          SQL_CP_ONE_PER_HENV: A single connection pool is supported for each environment
    \row
      \li SQL_ATTR_ODBC_VERSION
      \li SQL_OV_ODBC3: The driver should act as a ODBC 3.x driver\br
          SQL_OV_ODBC2: The driver should act as a ODBC 2.x driver (default)
    \endtable
    For more detailed information about the connect options please refer
    to the \l {https://learn.microsoft.com/en-us/sql/odbc/reference/syntax/sqlsetconnectattr-function}
    {SQLSetConnectAttr()} ODBC documentation.

    \section3 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 \l http://www.unixodbc.org.
    You need the unixODBC header files and shared libraries.

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

    \snippet code/doc_src_sql-driver.qdoc 11

    \section3 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:

    \snippet code/doc_src_sql-driver.qdoc 12

    \target QPSQL
    \section2 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 \l http://www.postgresql.org.

    \section3 Timestamp Support
    Since Qt 6.8, QDateTime values are converted to UTC before insertion
    and back from UTC during retrieval. To make this work, the driver sets
    the connection time zone to UTC during open() (SET TIME ZONE 'UTC').
    Although PostgreSQL has the `timestamptz` column type, the timezone
    used during insertion is not preserved and therefore all retrieved
    QDateTime values are UTC.

    \section3 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.

    \section3 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:

    \snippet code/doc_src_sql-driver.qdoc 39

    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:

    \snippet code/doc_src_sql-driver.qdoc 40

    \section3 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 \c true will
    have no effect.

    \warning 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:

    \snippet code/doc_src_sql-driver.qdoc 35

    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:

    \snippet code/doc_src_sql-driver_snippet.cpp 36

    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:

    \snippet code/doc_src_sql-driver.cpp 37

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

    \note Some methods of QSqlDatabase like tables(), primaryIndex()
    implicitly execute SQL queries, so these also cannot be used while
    navigating the results of forward-only query.

    \note QPSQL will print the following warning if it detects a loss of
    query results:

    \snippet code/doc_src_sql-driver.qdoc 38

    \section3 Connection options
    The Qt PostgreSQL plugin honors all connection options specified in the
    \l {https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PARAMKEYWORDS}
    {connect()} PostgreSQL documentation.

    \section3 How to Build the QPSQL Plugin on Unix and \macos

    You need the PostgreSQL client library and headers installed.

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

    \snippet code/doc_src_sql-driver.qdoc 13

    \section3 How to Build the QPSQL Plugin on Windows

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

    \snippet code/doc_src_sql-driver.qdoc 15

    Users of MinGW may wish to consult the following online document:
    \l{http://www.postgresql.org/docs/current/static/installation-platform-notes.html#INSTALLATION-NOTES-MINGW}{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.

    \target QDB2
    \section2 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()).

    \section3 Connection options
    The Qt IBM DB2 plugin honors the following connection options:
    \table
    \header \li Attribute \li Possible value
    \row
      \li SQL_ATTR_ACCESS_MODE
      \li SQL_MODE_READ_ONLY: open the database in read-only mode\br
          SQL_MODE_READ_WRITE: open the database in read-write mode (default)
    \row
      \li SQL_ATTR_LOGIN_TIMEOUT
      \li Number of seconds to wait for the database connection
          during login (max: 32767, a value of 0 will wait forever)
    \endtable

    \section3 How to Build the QDB2 Plugin on Unix and \macos

    \snippet code/doc_src_sql-driver.qdoc 18

    \section3 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:

    \snippet code/doc_src_sql-driver.qdoc 20

    \target QSQLITE
    \section2 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 \c{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 \l{http://www.sqlite.org}.

    \section3 Timestamp Support
    SQLite does not have a special timestamp column type. A QDateTime is
    stored as string, formatted in Qt::ISODateWithMs and therefore the
    QDateTime timezone information is preserved during insertion and
    select.

    \section3 Connection options
    The Qt SQLite plugin honors the following connection options:
    \table
    \header \li Attribute \li Possible value
    \row
      \li QSQLITE_BUSY_TIMEOUT
      \li Busy handler timeout in milliseconds (val <= 0: disabled),
          see \l {https://www.sqlite.org/c3ref/busy_timeout.html}
          {SQLite documentation} for more information

    \row
      \li QSQLITE_USE_QT_VFS
      \li If set, the database is opened using Qt's VFS which allows to
          open databases using QFile. This way it can open databases from
          any read-write locations (e.g.android shared storage) but also
          from read-only resources (e.g. qrc or android assets). Be aware
          that when opening databases from read-only resources make sure
          you add QSQLITE_OPEN_READONLY attribute as well.
          Otherwise it will fail to open it.

    \row
      \li QSQLITE_OPEN_READONLY
      \li If set, the database is open in read-only mode which will fail
          if no database exists. Otherwise the database will be opened in
          read-write mode and created if the database file does not yet
          exist (default)
    \row
      \li QSQLITE_OPEN_URI
      \li The given filename is interpreted as an uri, see
          \l {https://www.sqlite.org/c3ref/open.html} {SQLITE_OPEN_URI}
    \row
      \li QSQLITE_ENABLE_SHARED_CACHE
      \li If set, the database is opened in
          \l {https://www.sqlite.org/sharedcache.html} {shared cache mode},
          otherwise in private cache mode
    \row
      \li QSQLITE_ENABLE_REGEXP
      \li If set, the plugin defines a function 'regex' which can be used
          in queries, QRegularExpression is used for evaluation of the regex query
    \row
      \li QSQLITE_NO_USE_EXTENDED_RESULT_CODES
      \li Disables the usage of the \l {https://www.sqlite.org/c3ref/extended_result_codes.html}
          {extended result code} feature in SQLite
    \row
      \li QSQLITE_ENABLE_NON_ASCII_CASE_FOLDING
      \li If set, the plugin replaces the functions 'lower' and 'upper' with
          QString functions for correct case folding of non-ascii characters
    \row
      \li QSQLITE_OPEN_NOFOLLOW
      \li If set, the database filename is not allowed to contain a symbolic link
    \endtable

    \section3 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 \c{-DFEATURE_system_sqlite=OFF} parameter to the
    \c qt-cmake command line.

    If you do not want to use the SQLite library included with Qt, you
    can pass \c{-DFEATURE_system_sqlite=ON} to the \c 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 \c $SQLITE with the directory where
    SQLite resides):

    \snippet code/doc_src_sql-driver.qdoc 21

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

    \snippet code/doc_src_sql-driver.qdoc 23

    \section3 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 \l{QSqlDatabase::setConnectOptions()} {setting the connect
    option} \c{QSQLITE_ENABLE_REGEXP} before \l{QSqlDatabase::open()} {the
    database connection is opened}. Then a SQL statement like "column REGEXP
    'pattern'" basically expands to the Qt code

    \snippet code/doc_src_sql-driver.cpp 34

    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 "\c{QSQLITE_ENABLE_REGEXP=10}" reduces the
    cache size to 10.

    \section3 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 \c{SQLITE_DEFAULT_FILE_FORMAT} define when building SQLite.

    \target QMIMER
    \section2 QMIMER for Mimer SQL version 11 and higher

    The Qt Mimer SQL plugin makes it possible to work with the Mimer SQL RDBMS.
    Mimer SQL provides small footprint, scalable and robust relational database
    solutions that conform to international ISO SQL standards. Mimer SQL is available
    on Windows, Linux, \macos, and OpenVMS as well as several embedded platforms like QNX, Android,
    and embedded Linux.

    Mimer SQL fully support Unicode. To work with Unicode data the column types National Character (NCHAR),
    National Character Varying (NVARCHAR), or National Character Large Object (NCLOB) must be used.
    For more information about Mimer SQL and unicode, see \l{https://developer.mimer.com/features/multilingual-support}

    \section3 Timestamp Support
    MimerSQL does not know anything about timezones and QDateTime
    is used without honoring the timezone at all.

    \note: This might change in the future.

    \section3 QMIMER Stored Procedure Support

    Mimer SQL have stored procedures according to the SQL standard (PSM) and
    the plugin fully support IN, OUT, INOUT parameters as well as resultset procedures.

    Example stored procedure with INOUT and OUT parameters:

    \snippet code/doc_src_sql-driver.qdoc 44

    Source code to access the INOUT and OUT values:

    \snippet code/doc_src_sql-driver.cpp 40

    \section3 How to Build the QMIMER Plugin on Unix and \macos

    You need the Mimer SQL header files and shared libraries. Get them by installing
    any of the Mimer SQL variants found at \l{https://developer.mimer.com}.


    \snippet code/doc_src_sql-driver.qdoc 31

    \section3 How to Build the QMIMER Plugin on Windows

    You need the Mimer SQL header files and shared libraries. Get them by installing
    any of the Mimer SQL variants found at \l{https://developer.mimer.com}.

    \snippet code/doc_src_sql-driver.qdoc 32

    \target QIBASE
    \section2 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.

    \section3 Timestamp Support
    Interbase stores timestamps in UTC without any timezone information.
    Due to this, the QDateTime is used without honoring the timezone at all.

    Since Firebird 4.0, the database supports timestamps with timezones. The
    timezone information is stored separately to the timestamp so it can
    be properly retrieved later on. See the Firebird
    \l {https://firebirdsql.org/file/documentation/chunk/en/refdocs/fblangref40/fblangref40-datatypes-datetime.html}
    {documentation} for more information about timestamp handling.

    \section3 Connection options
    The Qt Borland InterBase plugin honors the following connection options:
    \table
    \header \li Attribute \li Possible value
    \row
      \li ISC_DPB_SQL_ROLE_NAME
      \li Specifies the login role name
    \endtable


    \section3 How to Build the QIBASE Plugin
    \snippet code/doc_src_sql-driver.cpp 24

    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.

    \section3 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:

    \snippet code/doc_src_sql-driver.cpp 26

    \section3 How to Build the QIBASE Plugin on Unix and \macos

    The following assumes InterBase or Firebird is installed in
    \c{/opt/interbase}:

    If you are using InterBase:

    \snippet code/doc_src_sql-driver.qdoc 27

    Optionally, use the CMake variables \c{Interbase_INCLUDE_DIR} and
    \c{Interbase_LIBRARY} to specify the include path and library directly.

    \section3 How to Build the QIBASE Plugin on Windows

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

    If you are using InterBase:

    \snippet code/doc_src_sql-driver.qdoc 29

    Optionally, use the CMake variables \c{Interbase_INCLUDE_DIR} and
    \c{Interbase_LIBRARY} to specify the include path and library directly.

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

    \target troubleshooting
    \section1 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.

    If the compilation of a plugin succeeds but it cannot be loaded
    afterwards, check out the following steps to find out the culprit:

    \list 1
    \li Ensure that the plugin is in the correct directory. You can use
        QApplication::libraryPaths() to determine where Qt looks for plugins.
    \li Ensure that the client libraries of the DBMS are available on the
        system. On Unix, run the command \c{ldd} and pass the name of the
        plugin as parameter, for example \c{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 or
        \l{https://github.com/lucasg/Dependencies/releases}{Dependencies GUI}
        to find out the dependent libraries. With
        Qt Creator, you can update the \c PATH environment variable in the
        \gui Run section of the \gui Project panel to include the path to
        the folder containing the client libraries.
    \li When using MSVC, also make sure the plugin is built with the correct
        build type. Due to different MSVC runtimes for debug and release,
        a Qt debug build can't load a Qt release plugin and vice versa.
    \li Run the compiled Qt executable with the environment variable
        \l{Environment Variables Recognized by Qt}{QT_DEBUG_PLUGINS} set
        to get very verbose debug output when loading plugins.
    \li To retrieve possible debug messages from the SQL subsystem, enable
        the output by setting the environment variable \c{QT_LOGGING_RULES}
        to \c{qt.sql.*.debug=true}. Don't forget to enable the
        \l{Environment Variables Recognized by Qt}{console} when working on
        windows.
        See \l{Logging Rules} for a more detailed explanation on how to set
        logging rules.
    \endlist

    Make sure you have followed the guide to \l{Deploying Plugins}.

    \target development
    \section1 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 \c{SELECT},
    \c{UPDATE}, and \c{ALTER} \c{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 \l{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
    \c{QTDIR/qtbase/src/plugins/sqldrivers}.

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

    \snippet sqldatabase/sqldatabase.cpp 47
    \codeline
    \snippet sqldatabase/sqldatabase.cpp 48
*/
