QT learning notes QtSql

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Preface

Due to work needs , To implement a small data management system . Ben wanted to pick it up QT Skill ideas , Study QT Development interface management database operation .

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One 、QtSQL brief introduction

QtSQL The module realizes the access to the database ,Qt SQL Three different levels of API For developers . Including driver layer ,SQL API layer , User interface layer .

level	describe
Driver layer	The specific database and SQL The underlying bridging of interfaces , The support classes included are QSqlDriver、QSqlDriverCreator、QSqlDriverCreatorBase、QSqlDriverPlugin and QSqlResult
SQL API layer	QSqlDataBase Class provides a database access interface 、 Database connection operation ;QSqlQuery Class provides operations that interact with the database , Other support classes include QSqlError、QSqlField、QSqlTableModel and QSqlRecord
User interface layer	Provide mapping from database data to forms for data representation , The support classes included are QSqlQueryModel、QSqlTableModel and QSqlRelationalTableModel, These classes are based on Qt Model of / View structure design

Using this module requires the following two steps ：

1. In the project file, that is pro Add the following line to the file

QT += sql 

2. Add the following line in the header file using this module

#include <QtSql>

Two 、SQLite brief introduction

There are many kinds of relational databases , Common is Oracle ORacle And Microsoft. SQL Server, And open source MySQL. These are a little extravagant and bloated when used in small database management .
Qt Provides an in-process database SQLite. No installation required , Lightweight . Views are also supported 、 Triggers and transactions , Support nested SQL function .

3、 ... and 、QtSQL Use

3.1 Establish a database connection

Before doing database operations , You must first establish a connection to the database . Database communication is usually distinguished by the connection name rather than the database name . We can establish multiple connections for the same database .QSqlDataBase Class supports creating default connections , It has no name . The following is a code to create and open the default connection

     QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
     db.setHostName("bigblue");
     db.setDatabaseName("flightdb");
     db.setUserName("acarlson");
     db.setPassword("1uTbSbAs");
     bool ok = db.open();

As shown in the code above , Creating a database connection only instantiates one QSqlDatabase Class object , If you want to use it, you have to call open Function to open it . Static functions addDatabase The prototype is as follows ：

 QSqlDatabase::addDatabase(
 const QString &type, 
 const QString &connectionName = QLatin1String(defaultConnection)
 )

Parameters type Is the driver name , You can specify which databases to connect to ; Parameters connectionname Is the above connection name , If not specified , Then use the default connection name , Later, you can directly use the correlation function without parameters .QT The supported drivers are as follows ：

Driver name	DBMS
QDB2	IBM DB2 (version 7.1 and above)
QIBASE	Borland InterBase
QMYSQL	MySQL
QOCI	Oracle Call Interface Driver
QODBC	Open Database Connectivity (ODBC) - Microsoft SQL Server and other ODBC-compliant databases
QPSQL	PostgreSQL (versions 7.3 and above)
QSQLITE2	SQLite version 2
QSQLITE	SQLite version 3

After opening the database connection, we can operate the database through related functions , After using, you can close and remove the database through the following two functions

 QSqlDatabase::close()
 QSqlDatabase::removeDatabase()

3.2 perform SQL sentence

QSqlQuery Class provides an implementation SQL Statement and can traverse the returned result set of execution . After the database connection is established, you can use this class to execute the... Supported by the underlying database SQL sentence , All this method needs to do is create an instance object of this class , Then call the exec() function . as follows ：

     QSqlQuery query;
     query.exec("SELECT name, salary FROM employee WHERE salary > 50000");

3.1.1 Traversal result set

QSqlQuery Class provides an interface to get records , Calling exec After the function ,QSqlQuery The internal pointer of the class points to the position before the first record , We must call once QSqlQuery::next() Function to get the location of the first record . Then repeat the call next function , Until an error is returned .

     while (query.next()) {
         QString name = query.value(0).toString();
         int salary = query.value(1).toInt();
         qDebug() << name << salary;
     }

QSqlQuery::value() Function returns a property value of the current record . Fields It's from 0 Index started . We can go through QSqlQuery::at() Get the row index of the current record , adopt QSqlQuery::size() Get the total number of records , That is, the number of lines .
QSqlQuery::value() The return value of QVariant type , This type can be converted to many C++ and core Qt data type , Such as int, QString, and QByteArray. Different database types are automatically mapped to similar Qt type .

QSQLITE SQLite version 3 Data Types

QSQLITE SQLite version 3 data type	SQL type description	Recommended input (C++ or Qt data type)
NULL	NULL value.	NULL
INTEGER	Signed integer, stored in 8, 16, 24, 32, 48, or 64-bits depending on the magnitude of the value.	typedef qint8/16/32/64
REAL	64-bit floating point value.	By default mapping to QString
TEXT	Character string (UTF-8, UTF-16BE or UTF-16-LE).	Mapped to QString
CLOB	Character large string object	Mapped to QString
BLOB	The value is a BLOB of data, stored exactly as it was input.	Mapped to QByteArray

3.1.2 increase 、 Update and delete records

QSqlQuery You can perform a variety of SQL sentence . More than select, Such as insert,update,delete.

1.insert

     QSqlQuery query;
     query.exec("INSERT INTO employee (id, name, salary) "
                "VALUES (1001, 'Thad Beaumont', 65000)");

You can insert multiple records using placeholder operations ,Qt Two kinds of placeholder syntax are supported ： Famous occupation and positioning occupation . Respectively
Famous occupation, i.e Oracle grammar

     QSqlQuery query;
     query.prepare("INSERT INTO employee (id, name, salary) "
                   "VALUES (:id, :name, :salary)");
     query.bindValue(":id", 1001);
     query.bindValue(":name", "Thad Beaumont");
     query.bindValue(":salary", 65000);
     query.exec();

Positioning space occupying means ODBC grammar

     QSqlQuery query;
     query.prepare("INSERT INTO employee (id, name, salary) "
                   "VALUES (?, ?, ?)");
     query.addBindValue(1001);
     query.addBindValue("Thad Beaumont");
     query.addBindValue(65000);
     query.exec();

2.update

     QSqlQuery query;
     query.exec("UPDATE employee SET salary = 70000 WHERE id = 1003");

3.delete

     QSqlQuery query;
     query.exec("DELETE FROM employee WHERE id = 1007"); 

3.1.3 The transaction operations

Transactions can be used to ensure that complex operations are atomic （ for example , Find foreign keys and create records ）, Or provide a way to eliminate complex changes in the middle .
have access to QSQLDABASE:：transaction（） Start transaction , Then it is to be executed in the transaction context SQL command , And finally QSQLDABASE:：commit（） or QSQLDABASE:：rollback（）.
When using transaction operations , The transaction must be started before chuangjie query . Examples are as follows ：

     QSqlDatabase::database().transaction();
     QSqlQuery query;
     query.exec("SELECT id FROM employee WHERE name = 'Torild Halvorsen'");
     if (query.next()) {
         int employeeId = query.value(0).toInt();
         query.exec("INSERT INTO project (id, name, ownerid) "
                    "VALUES (201, 'Manhattan Project', "
                    + QString::number(employeeId) + ')');
     }
     QSqlDatabase::database().commit();

3.3SQL Model class

Qt At the same time, three model classes are provided in the user interface layer to operate the database ,QSqlQueryModel、QSqlTableModel and QSqlRelationalTableModel. These three classes all inherit from QAbstractTableModel , Therefore, the data can be easily passed QListView and QTableView On display in UI In the interface ; In addition, based on the model / View structure , We can also easily change the data source without changing the code .

QSqlQueryModel

Provides a SQL Read only model of the query

     QSqlQueryModel model;
     model.setQuery("SELECT * FROM employee");

     for (int i = 0; i < model.rowCount(); ++i) {
         int id = model.record(i).value("id").toInt();
         QString name = model.record(i).value("name").toString();
         qDebug() << id << name;
     }

QSqlTableModel

Provides a leaflet based SQL Read write model of table

     QSqlTableModel model;
     model.setTable("employee");
     model.setFilter("salary > 50000");
     model.setSort(2, Qt::DescendingOrder);
     model.select();

     for (int i = 0; i < model.rowCount(); ++i) {
         QString name = model.record(i).value("name").toString();
         int salary = model.record(i).value("salary").toInt();
         qDebug() << name << salary;
     }

Use the model , Can replace write SQL Statement to operate , If we can pass record() Or a row of records in the table , adopt setRecord() Modify the record . After all modifications on the model are completed , Use submitall perhaps submit To submit , Update the database .

     for (int i = 0; i < model.rowCount(); ++i) {
         QSqlRecord record = model.record(i);
         double salary = record.value("salary").toInt();
         salary *= 1.1;
         record.setValue("salary", salary);
         model.setRecord(i, record);
     }
     model.submitAll();

Set editing policy
The editing policy indicates whether the data modification made by the user in the view is fully applied to the database . The value range is

Constant	value	describe
QSqlTableModel::OnFieldChange	0	All changes to the model will be immediately updated in the database
QSqlTableModel::OnRowChange	1	The modification of the current line takes effect when the user selects another different line
QSqlTableModel::OnManualSubmit	2	All modifications are temporarily stored in the model , Until the call submitAll() perhaps revertAll()

model->setEditStrategy(QSqlTableModel::OnManualSubmit);

QSqlRelationalTableModel

be based on QSqlTableModel An extension of , Support foreign keys .

3.4 Display data through views

QSqlQueryModel、QSqlTableModel and QSqlRelationalTableModel Classes can be used as data sources in Qt In the view class of , Such as QListView、QTableView and QTreeView And so on . among , The most common is QTableView, Because this view is most suitable for representing two-dimensional SQL Operating results .
If the model is readable and writable , The view will allow the user to edit fields , We can disable editing by calling .

     view->setEditTriggers(QAbstractItemView::NoEditTriggers);

We can use one model for multiple views , When the model is updated and edited in a view , The other views will update .
When using the insert row and delete row operations of the model , Special annotation will be made on the view , Until submission .
The view class can display a horizontal header and a vertical header . The horizontal header displays a column name above each column , By default , The column name is the field name of the database table , Can pass setHeaderData() Function to modify the column name . The vertical header displays the line number of each line on the leftmost side .

     model->setHeaderData(0, Qt::Horizontal, QObject::tr("ID"));
     model->setHeaderData(1, Qt::Horizontal, QObject::tr("Name"));
     model->setHeaderData(2, Qt::Horizontal, QObject::tr("City"));
     model->setHeaderData(3, Qt::Horizontal, QObject::tr("Country"));

Create a data aware form

At some point , We want to get and update the data of specific rows or columns in the database table through a form , You can use it QDataWidgetMapper class , For details, please refer to the official BOOKS Example . I haven't used this function yet .