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+****************************************************************************/
+/*!
+\group database
+\title Database Classes
+\brief Database related classes, e.g. for SQL databases.
+*/
+/*!
+\page sql-programming.html
+\title SQL Programming
+\brief Database integration for Qt applications.
+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.
+\section1 Topics:
+\list
+\o \l{Database Classes}
+\o \l{Connecting to Databases}
+\list
+\o \l{SQL Database Drivers}
+\endlist
+\o \l{Executing SQL Statements}
+\list
+\o \l{Recommended Use of Data Types in Databases}
+\endlist
+\o \l{Using the SQL Model Classes}
+\o \l{Presenting Data in a Table View}
+\o \l{Creating Data-Aware Forms}
+\endlist
+\section1 Database Classes
+These classes provide access to SQL databases.
+\annotatedlist database
+The SQL classes are divided into three layers:
+\section2 Driver Layer
+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.
+\section2 SQL API Layer
+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.
+\section2 User Interface Layer
+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
+\l{Model/View Programming}{model/view framework}.
+Note that to use any of these classes, a QCoreApplication object
+must have been instantiated first.
+*/
+/*!
+\page sql-connecting.html
+\title Connecting to Databases
+\contentspage SQL Programming
+\nextpage Executing SQL Statements
+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().
+*/
+/*!
+\page sql-sqlstatements.html
+\title Executing SQL Statements
+\previouspage Connecting to Databases
+\contentspage SQL Programming
+\nextpage Using the SQL Model Classes
+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.
+For an overview of the recommended types used with Qt supported
+Databases, please refer to \l{Recommended Use of Data Types in Databases}{this table}.
+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
+*/
+/*!
+\page sql-model.html
+\title Using the SQL Model Classes
+\previouspage Executing SQL Statements
+\contentspage SQL Programming
+\nextpage Presenting Data in a Table View
+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.
+*/
+/*!
+\page sql-presenting.html
+\title Presenting Data in a Table View
+\previouspage Using the SQL Model Classes
+\contentspage SQL Programming
+\nextpage Creating Data-Aware Forms
+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.
+*/
+/*!
+\page sql-forms.html
+\title Creating Data-Aware Forms
+\previouspage Presenting Data in a Table View
+\contentspage SQL Programming
+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.
+*/