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/*!

    \page qt4-sql.html

    \title The Qt 4 Database GUI Layer

    \contentspage {What's New in Qt 4}{Home}

    \previouspage Cross-Platform Accessibility Support in Qt 4

    \nextpage The Network Module in Qt 4

    The GUI layer of the SQL module in Qt 4 has been entirely

    redesigned to work with \l{qt4-interview.html}{Interview} (Qt's

    new model/view classes). It consists of three model classes

    (QSqlQueryModel, QSqlTableModel, and QSqlRelationalTableModel)

    that can be used with Qt's view classes, notably QTableView.

    \section1 General Overview

    The Qt 4 SQL classes are divided into three layers:

    \list

    \o The database drivers

    \o The core SQL classes

    \o The GUI classes

    \endlist

    The database drivers and the core SQL classes are mostly the same

    as in Qt 3. The database item models are new with Qt 4; they

    inherit from QAbstractItemModel and make it easy to present data

    from a database in a view class such as QListView, QTableView,

    and QTreeView.

    The philosophy behind the Qt 4 SQL module is that it should be

    possible to use database models for rendering and editing data

    just like any other item models. By changing the model at

    run-time, you can decide whether you want to store your data in

    an SQL database or in, say, an XML file. This generic approach

    has the additional benefit that you don't need to know anything

    about SQL to display and edit data.

    The Qt 4 SQL module includes three item models:

    \list

    \o QSqlQueryModel is a read-only model based on an arbitrary

       SQL query.

    \o QSqlTableModel is a read-write model that works on a single

       table.

    \o QSqlRelationalTableModel is a QSqlTableModel subclass with

       foreign key support.

    \endlist

    Combined with Qt's view classes and Qt's default delegate class

    (QItemDelegate), the models offer a very powerful mechanism for

    accessing databases. For finer control on the rendering of the

    fields, you can subclass one of the predefined models, or even

    QAbstractItemDelegate or QItemDelegate if you need finer control.

    You can also perform some customizations without subclassing. For

    example, you can sort a table using QSqlTableModel::sort(), and

    you can initialize new rows by connecting to the

    QSqlTableModel::primeInsert() signal.

    One nice feature supported by the read-write models is the

    possibility to perform changes to the item model without

    affecting the database until QSqlTableModel::submitAll() is

    called. Changes can be dropped using QSqlTableModel::revertAll().

    The new classes perform advantageously compared to the SQL

    module's GUI layer in Qt 3. Speed and memory improvements in the

    tool classes (especially QVariant, QString, and QMap) and in the

    SQL drivers contribute to making Qt 4 database applications more

    snappy.

    See the \l QtSql module overview for a more complete introduction

    to Qt's SQL classes.

    \section1 Example Code

    The simplest way to present data from a database is to simply

    combine a QSqlQueryModel with a QTableView:

    \snippet doc/src/snippets/code/doc_src_qt4-sql.qdoc 0

    To present the contents of a single table, we can use

    QSqlTableModel instead:

    \snippet doc/src/snippets/code/doc_src_qt4-sql.qdoc 1

    In practice, it's common that we need to customize the rendering

    of a field in the database. In that case, we can create our own

    model based on QSqlQueryModel. The next code snippet shows a

    custom model that prepends '#' to the value in field 0 and

    converts the value in field 2 to uppercase:

    \snippet examples/sql/querymodel/customsqlmodel.h 0

    \codeline

    \snippet examples/sql/querymodel/customsqlmodel.cpp 0

    It is also possible to subclass QSqlQueryModel to add support for

    editing. This is done by reimplementing

    QAbstractItemModel::flags() to specify which database fields are

    editable and QAbstractItemModel::setData() to modify the

    database. Here's an example of a setData() reimplementation that

    changes the first or last name of a person:

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

    It relies on helper functions called \c setFirstName() and

    \c setLastName(), which execute an \c{update}. Here's

    \c setFirstName():

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

    See Qt's \c examples/sql directory for more examples.

    \section1 Comparison with Qt 3

    The core SQL database classes haven't changed so much since Qt 3.

    Here's a list of the main changes:

    \list

    \o QSqlDatabase is now value-based instead of pointer-based.

    \o QSqlFieldInfo and QSqlRecordInfo has been merged into

       QSqlField and QSqlRecord.

    \o The SQL query generation has been moved into the drivers. This

       makes it possible to use non-standard SQL extensions. It also

       opens the door to non-SQL databases.

    \endlist

    The GUI-related database classes have been entirely redesigned.

    The QSqlCursor abstraction has been replaced with QSqlQueryModel

    and QSqlTableModel; QSqlEditorFactory is replaced by

    QAbstractItemDelegate; QDataTable is replaced by QTableView. The

    old classes are part of the \l{Qt3Support} library to aid

    porting to Qt 4.

*/