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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

    \nextpage Connecting to Databases

    \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

    \previouspage 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