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/*!
    \module QtSql
    \title QtSql Module
    \contentspage Qt's Modules
    \previouspage QtScript
    \nextpage QtSvg
    \ingroup modules

    \brief The QtSql module helps you provide seamless database
    integration to your Qt applications.

    The SQL classes are divided into three layers:

    \table
    \header \o Layer     \o Description
    \row    \o \bold{Driver Layer}
            \o This comprises the classes QSqlDriver,
               QSqlDriverCreator<T>, QSqlDriverCreatorBase,
               QSqlDriverPlugin, and QSqlResult. This layer provides the
               low-level bridge between the specific databases and the
               SQL API layer. See \l{SQL Database Drivers} for more
               information.
    \row    \o \bold{SQL API Layer}
            \o These classes provide access to databases. Connections
               are made using the QSqlDatabase class. Database
               interaction is achieved by using the QSqlQuery class.
               In addition to QSqlDatabase and QSqlQuery, the SQL API
               layer is supported by QSqlError, QSqlField, QSqlIndex,
               and QSqlRecord.

    \row    \o \bold{User Interface Layer}
            \o These classes link the data from a database to
               data-aware widgets. They include QSqlQueryModel,
               QSqlTableModel, and QSqlRelationalTableModel. These
               classes are designed to work with Qt's model/view
               framework.
    \endtable

    Note that to use any of these classes, a QCoreApplication object
    must have been instantiated first. To include the definitions of
    the module's classes, use the following directive:

    \snippet doc/src/snippets/code/doc_src_qtsql.qdoc 0

    To link against the module, add this line to your \l qmake \c
    .pro file:

    \snippet doc/src/snippets/code/doc_src_qtsql.qdoc 1

    The QtSql module is part of the \l{Qt Full Framework Edition} and the
    \l{Open Source Versions of Qt}.

    This overview assumes that you have at least a basic knowledge of
    SQL. You should be able to understand simple \c SELECT, \c
    INSERT, \c UPDATE, and \c DELETE statements. Although the \l
    QSqlTableModel class provides an interface to database browsing
    and editing that does not require a knowledge of SQL, a basic
    understanding of SQL is highly recommended. A standard text
    covering SQL databases is \e {An Introduction to Database Systems}
    (7th Ed.) by C. J. Date, ISBN 0201385902.

    Topics:

    \tableofcontents

    \section1 Connecting to Databases

    To access a database with QSqlQuery or QSqlQueryModel, create and
    open one or more database connections. Database connections are
    normally identified by connection name, \e{not} by database name.
    You can have multiple connections to the same database.
    QSqlDatabase also supports the concept of a \e{default}
    connection, which is an unnamed connection. When calling QSqlQuery
    or QSqlQueryModel member functions that take a connection name
    argument, if you don't pass a connection name, the default
    connection will be used. Creating a default connection is
    convenient when your application only requires one database
    connection.

    Note the difference between creating a connection and opening it.
    Creating a connection involves creating an instance of class
    QSqlDatabase. The connection is not usable until it is opened. The
    following snippet shows how to create a \e{default} connection
    and then open it:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 26

    The first line creates the connection object, and the last line
    opens it for use. In between, we initialize some connection
    information, including the \l{QSqlDatabase::setDatabaseName()}
    {database name}, the \l{QSqlDatabase::setHostName()} {host name},
    the \l{QSqlDatabase::setUserName()} {user name}, and the
    \l{QSqlDatabase::setPassword()} {password}. In this case, we are
    connecting to the MySQL database \c{flightdb} on the host
    \c{bigblue}. The \c{"QMYSQL"} argument to
    \l{QSqlDatabase::addDatabase()} {addDatabase()} specifies the type
    of database driver to use for the connection. The set of database
    drivers included with Qt are shown in the table of \l{SQL Database
    Drivers#Supported Databases} {supported database drivers}. 

    The connection in the snippet will be the \e{default} connection,
    because we don't pass the second argument to
    \l{QSqlDatabase::addDatabase()} {addDatabase()}, which is the
    connection name. For example, here we establish two MySQL database
    connections named \c{"first"} and \c{"second"}:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 27

    After these connections have been initialized, \l{QSqlDatabase::}
    {open()} for each one to establish the live connections. If the
    \l{QSqlDatabase::} {open()} fails, it returns false. In that case,
    call QSqlDatabase::lastError() to get error information.

    Once a connection is established, we can call the static function
    QSqlDatabase::database() from anywhere with a connection name to
    get a pointer to that database connection. If we don't pass a
    connection name, it will return the default connection. For
    example:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 28
    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 29
    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 30

    To remove a database connection, first close the database using
    QSqlDatabase::close(), then remove it using the static method
    QSqlDatabase::removeDatabase().

    \section1 Executing SQL Statements

    The QSqlQuery class provides an interface for executing SQL
    statements and navigating through the result set of a query.

    The QSqlQueryModel and QSqlTableModel classes described in the
    next section provide a higher-level interface for accessing
    databases. If you are unfamiliar with SQL, you might want to skip
    directly to the next section (\l{Using the SQL Model Classes}).

    \section2 Executing a Query

    To execute an SQL statement, simply create a QSqlQuery object and
    call QSqlQuery::exec() like this:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 31

    The QSqlQuery constructor accepts an optional QSqlDatabase object
    that specifies which database connection to use. In the example
    above, we don't specify any connection, so the default connection
    is used.

    If an error occurs, \l{QSqlQuery::exec()}{exec()} returns false.
    The error is then available as QSqlQuery::lastError().

    \section2 Navigating the Result Set

    QSqlQuery provides access to the result set one record at a time.
    After the call to \l{QSqlQuery::exec()}{exec()}, QSqlQuery's
    internal pointer is located one position \e{before} the first
    record. We must call QSqlQuery::next() once to advance to the
    first record, then \l{QSqlQuery::next()}{next()} again repeatedly
    to access the other records, until it returns false. Here's a
    typical loop that iterates over all the records in order:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 32

    The QSqlQuery::value() function returns the value of a field in
    the current record. Fields are specified as zero-based indexes.
    QSqlQuery::value() returns a QVariant, a type that can hold
    various C++ and core Qt data types such as \c int, QString, and
    QByteArray. The different database types are automatically mapped
    into the closest Qt equivalent. In the code snippet, we call
    QVariant::toString() and QVariant::toInt() to convert
    variants to QString and \c int.

    You can iterate back and forth using QSqlQuery::next(),
    QSqlQuery::previous(), QSqlQuery::first(), QSqlQuery::last(), and
    QSqlQuery::seek(). The current row index is returned by
    QSqlQuery::at(), and the total number of rows in the result set
    is avaliable as QSqlQuery::size() for databases that support it.

    To determine whether a database driver supports a given feature,
    use QSqlDriver::hasFeature(). In the following example, we call
    QSqlQuery::size() to determine the size of a result set of
    the underlying database supports that feature; otherwise, we
    navigate to the last record and use the query's position to tell
    us how many records there are.

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 33

    If you iterate through a result set only using next() and seek()
    with positive values, you can call
    QSqlQuery::setForwardOnly(true) before calling exec(). This is an
    easy optimization that will speed up the query significantly when
    operating on large result sets.

    \section2 Inserting, Updating, and Deleting Records

    QSqlQuery can execute arbitrary SQL statements, not just
    \c{SELECT}s. The following example inserts a record into a table
    using \c{INSERT}:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 34

    If you want to insert many records at the same time, it is often
    more efficient to separate the query from the actual values being
    inserted. This can be done using placeholders. Qt supports two
    placeholder syntaxes: named binding and positional binding.
    Here's an example of named binding:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 35

    Here's an example of positional binding:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 36

    Both syntaxes work with all database drivers provided by Qt. If
    the database supports the syntax natively, Qt simply forwards the
    query to the DBMS; otherwise, Qt simulates the placeholder syntax
    by preprocessing the query. The actual query that ends up being
    executed by the DBMS is available as QSqlQuery::executedQuery().

    When inserting multiple records, you only need to call
    QSqlQuery::prepare() once. Then you call
    \l{QSqlQuery::bindValue()}{bindValue()} or
    \l{QSqlQuery::addBindValue()}{addBindValue()} followed by
    \l{QSqlQuery::exec()}{exec()} as many times as necessary.

    Besides performance, one advantage of placeholders is that you
    can easily specify arbitrary values without having to worry about
    escaping special characters.

    Updating a record is similar to inserting it into a table:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 37

    You can also use named or positional binding to associate
    parameters to actual values.

    Finally, here's an example of a \c DELETE statement:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 38

    \section2 Transactions

    If the underlying database engine supports transactions,
    QSqlDriver::hasFeature(QSqlDriver::Transactions) will return
    true. You can use QSqlDatabase::transaction() to initiate a
    transaction, followed by the SQL commands you want to execute
    within the context of the transaction, and then either
    QSqlDatabase::commit() or QSqlDatabase::rollback(). When
    using transactions you must start the transaction before you
    create your query.

    Example:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 39

    Transactions can be used to ensure that a complex operation is
    atomic (for example, looking up a foreign key and creating a
    record), or to provide a means of canceling a complex change in
    the middle.

    \omit
    It would be useful to mention transactions, and the fact that
    some databases don't support them.
    \endomit

    \section1 Using the SQL Model Classes

    In addition to QSqlQuery, Qt offers three higher-level classes
    for accessing databases. These classes are QSqlQueryModel,
    QSqlTableModel, and QSqlRelationalTableModel.

    \table
    \row \o QSqlQueryModel
         \o A read-only model based on an arbitrary SQL query.
    \row \o QSqlTableModel
         \o A read-write model that works on a single table.
    \row \o QSqlRelationalTableModel
         \o A QSqlTableModel subclass with foreign key support.
    \endtable

    These classes derive from QAbstractTableModel (which in turn
    inherits from QAbstractItemModel) and make it easy to present
    data from a database in an item view class such as QListView and
    QTableView. This is explained in detail in the \l{Presenting Data
    in a Table View} section.

    Another advantage of using these classes is that it can make your
    code easier to adapt to other data sources. For example, if you
    use QSqlTableModel and later decide to use XML files to store
    data instead of a database, it is essentially just a matter of
    replacing one data model with another.

    \section2 The SQL Query Model

    QSqlQueryModel offers a read-only model based on an SQL query.

    Example:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 40

    After setting the query using QSqlQueryModel::setQuery(), you can
    use QSqlQueryModel::record(int) to access the individual records.
    You can also use QSqlQueryModel::data() and any of the other
    functions inherited from QAbstractItemModel.

    There's also a \l{QSqlQueryModel::setQuery()}{setQuery()}
    overload that takes a QSqlQuery object and operates on its result
    set. This enables you to use any features of QSqlQuery to set up
    the query (e.g., prepared queries).

    \section2 The SQL Table Model

    QSqlTableModel offers a read-write model that works on a single
    SQL table at a time.

    Example:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 41

    QSqlTableModel is a high-level alternative to QSqlQuery for
    navigating and modifying individual SQL tables. It typically
    results in less code and requires no knowledge of SQL syntax.

    Use QSqlTableModel::record() to retrieve a row in the table, and
    QSqlTableModel::setRecord() to modify the row. For example, the
    following code will increase every employee's salary by 10 per
    cent:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 42

    You can also use QSqlTableModel::data() and
    QSqlTableModel::setData(), which are inherited from
    QAbstractItemModel, to access the data. For example, here's how
    to update a record using
    \l{QSqlTableModel::setData()}{setData()}:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 43

    Here's how to insert a row and populate it:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 44

    Here's how to delete five consecutive rows:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 45
    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 46

    The first argument to QSqlTableModel::removeRows() is the index
    of the first row to delete.

    When you're finished changing a record, you should always call
    QSqlTableModel::submitAll() to ensure that the changes are
    written to the database.

    When and whether you actually \e need to call submitAll() depends
    on the table's \l{QSqlTableModel::editStrategy()}{edit strategy}.
    The default strategy is QSqlTableModel::OnRowChange, which
    specifies that pending changes are applied to the database when
    the user selects a different row. Other strategies are
    QSqlTableModel::OnManualSubmit (where all changes are cached in
    the model until you call submitAll()) and
    QSqlTableModel::OnFieldChange (where no changes are cached).
    These are mostly useful when QSqlTableModel is used with a view.

    QSqlTableModel::OnFieldChange seems to deliver the promise that
    you never need to call submitAll() explicitly. There are two
    pitfalls, though:

    \list
    \o Without any caching, performance may drop significantly.
    \o If you modify a primary key, the record might slip through
       your fingers while you are trying to populate it.
    \endlist

    \section2 The SQL Relational Table Model

    QSqlRelationalTableModel extends QSqlTableModel to provide
    support for foreign keys. A foreign key is a 1-to-1 mapping
    between a field in one table and the primary key field of another
    table. For example, if a \c book table has a field called \c
    authorid that refers to the author table's \c id field, we say
    that \c authorid is a foreign key.

    \table
    \row \o \inlineimage noforeignkeys.png
         \o \inlineimage foreignkeys.png
    \endtable

    The screenshot on the left shows a plain QSqlTableModel in a
    QTableView. Foreign keys (\c city and \c country) aren't resolved
    to human-readable values. The screenshot on the right shows a
    QSqlRelationalTableModel, with foreign keys resolved into
    human-readable text strings.

    The following code snippet shows how the QSqlRelationalTableModel
    was set up:

    \snippet examples/sql/relationaltablemodel/relationaltablemodel.cpp 0
    \codeline
    \snippet examples/sql/relationaltablemodel/relationaltablemodel.cpp 1
    \snippet examples/sql/relationaltablemodel/relationaltablemodel.cpp 2

    See the QSqlRelationalTableModel documentation for details.

    \section1 Presenting Data in a Table View

    The QSqlQueryModel, QSqlTableModel, and QSqlRelationalTableModel
    classes can be used as a data source for Qt's view classes such
    as QListView, QTableView, and QTreeView. In practice, QTableView
    is by far the most common choice, because an SQL result set is
    essentially a two-dimensional data structure.

    \image relationaltable.png A table view displaying a QSqlTableModel

    The following example creates a view based on an SQL data model:

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 17
    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 18
    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 19

    If the model is a read-write model (e.g., QSqlTableModel), the
    view lets the user edit the fields. You can disable this by
    calling

    \snippet doc/src/snippets/sqldatabase/sqldatabase.cpp 20

    You can use the same model as a data source for multiple views.
    If the user edits the model through one of the views, the other
    views will reflect the changes immediately. The
    \l{sql/tablemodel}{Table Model} example shows how it works.

    View classes display a header at the top to label the columns. To
    change the header texts, call
    \l{QAbstractItemModel::setHeaderData()}{setHeaderData()} on the
    model. The header's labels default to the table's field names.
    For example:

    \snippet examples/sql/relationaltablemodel/relationaltablemodel.cpp 3

    QTableView also has a vertical header on the left with numbers
    identifying the rows. If you insert rows programmatically using
    QSqlTableModel::insertRows(), the new rows will be marked with an
    asterisk (*) until they are submitted using
    \l{QSqlTableModel::submitAll()}{submitAll()} or automatically
    when the user moves to another record (assuming the
    \l{QSqlTableModel::EditStrategy}{edit strategy} is
    QSqlTableModel::OnRowChange).

    \image insertrowinmodelview.png Inserting a row in a model

    Likewise, if you remove rows using
    \l{QSqlTableModel::removeRows()}{removeRows()}, the rows will be
    marked with an exclamation mark (!) until the change is
    submitted.

    The items in the view are rendered using a delegate. The default
    delegate, QItemDelegate, handles the most common data types (\c
    int, QString, QImage, etc.). The delegate is also responsible for
    providing editor widgets (e.g., a combobox) when the user starts
    editing an item in the view. You can create your own delegates by
    subclassing QAbstractItemDelegate or QItemDelegate. See
    \l{Model/View Programming} for more information.

    QSqlTableModel is optimized to operate on a single table at a
    time. If you need a read-write model that operates on an
    arbitrary result set, you can subclass QSqlQueryModel and
    reimplement \l{QAbstractItemModel::flags()}{flags()} and
    \l{QAbstractItemModel::setData()}{setData()} to make it
    read-write. The following two functions make fields 1 and 2 of a
    query model editable:

    \snippet examples/sql/querymodel/editablesqlmodel.cpp 0
    \codeline
    \snippet examples/sql/querymodel/editablesqlmodel.cpp 1

    The setFirstName() helper function is defined as follows:

    \snippet examples/sql/querymodel/editablesqlmodel.cpp 2

    The setLastName() function is similar. See the
    \l{sql/querymodel}{Query Model} example for the complete source code.

    Subclassing a model makes it possible to customize it in many
    ways: You can provide tooltips for the items, change the
    background color, provide calculated values, provide different
    values for viewing and editing, handle null values specially, and
    more. See \l{Model/View Programming} as well as the \l
    QAbstractItemView reference documentation for details.

    If all you need is to resolve a foreign key to a more
    human-friendly string, you can use QSqlRelationalTableModel. For
    best results, you should also use QSqlRelationalDelegate, a
    delegate that provides combobox editors for editing foreign keys.

    \image relationaltable.png Editing a foreign key in a relational table

    The \l{sql/relationaltablemodel}{Relational Table Model} example
    illustrates how to use QSqlRelationalTableModel in conjunction with
    QSqlRelationalDelegate to provide tables with foreign key
    support.

    \section1 Creating Data-Aware Forms

    Using the SQL models described above, the contents of a database can
    be presented to other model/view components. For some applications,
    it is sufficient to present this data using a standard item view,
    such as QTableView. However, users of record-based applications often
    require a form-based user interface in which data from a specific
    row or column in a database table is used to populate editor widgets
    on a form.

    Such data-aware forms can be created with the QDataWidgetMapper class,
    a generic model/view component that is used to map data from a model
    to specific widgets in a user interface.

    QDataWidgetMapper operates on a specific database table, mapping items
    in the table on a row-by-row or column-by-column basis. As a result,
    using QDataWidgetMapper with a SQL model is as simple as using it with
    any other table model.

    \image qdatawidgetmapper-simple.png

    The \l{demos/books}{Books} demonstration shows how information can
    be presented for easy access by using QDataWidgetMapper and a set of
    simple input widgets.
*/