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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. */