Difference between revisions of "How to Store and Retrieve Image on SQLite"

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(various minor fixes)
(expand code snippets into a full working example)
 
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{{LangSwitch}}
 
{{LangSwitch}}
  
Images or any files can be stored in a database. Here is one way to do it using the following steps:
+
Images (or any other binary data, from files or otherwise) can be stored in a database.
 +
One way to do it is converting the data into a QByteArray, and insert it into the database as a [[wikipedia:Binary large object|Binary Large OBject]] (BLOB).
  
# Read the system file into a QByteArray.
+
Below is a full example of such a workflow, including reading the image back from the database and displaying it:
# Store QByteArray as a Binary Large Object in database.
 
  
For example:
+
<source lang="cpp">
 +
#include <QtSql>
 +
#include <QtWidgets>
  
<code>
+
int main( int argc, char *argv[] )
QFile file(fileName);
+
{
if (!file.open(QIODevice::ReadOnly)) return;
+
    QApplication app( argc, argv );
QByteArray byteArray = file.readAll();
 
  
QSqlQuery query;
+
    // Set up database
query.prepare("INSERT INTO imgtable (imgdata) VALUES (?)");
+
    QString dbName( "myDatabase.db3" );
query.addBindValue(byteArray);
+
    QFile::remove( dbName ); // delete sqlite file if it exists from a previous run
query.exec();
+
    QSqlDatabase db = QSqlDatabase::addDatabase( "QSQLITE" );
</code>
+
    db.setDatabaseName( dbName );
 +
    db.open();
 +
    QSqlQuery query = QSqlQuery( db );
 +
    query.exec( "CREATE TABLE IF NOT EXISTS imgTable ( filename TEXT, imagedata BLOB )" );
  
Now, the image/file can be retrieved like any other data
+
    // Generate an image (in this case a screenshot) and store it in a QByteArray
<code>
+
    QScreen *screen = app.primaryScreen();
QSqlQuery query("SELECT imgdata FROM imgtable");
+
    QPixmap inPixmap = screen->grabWindow( 0 );
query.next();
+
    QByteArray inByteArray;
QByteArray array = query.value(0).toByteArray();
+
    QBuffer inBuffer( &inByteArray );
</code>
+
    inBuffer.open( QIODevice::WriteOnly );
 +
    inPixmap.save( &inBuffer, "PNG" ); // write inPixmap into inByteArray in PNG format
  
Creating a QPixmap from QByteArray :
+
    // Alternatively, load an image file directly into a QByteArray
<code>
+
    // QFile file("screenshot.png");
QPixmap pixmap = QPixmap();
+
    // if (!file.open(QIODevice::ReadOnly)) return;
pixmap.loadFromData(array);
+
    // QByteArray inByteArray = file.readAll();
</code>
 
  
It is done. Now the pixmap can be used in a {{class|QPushButton}} as icon or in label etc.
+
    // Insert image bytes into the database
 +
    // Note: make sure to wrap the :placeholder in parenthesis
 +
    query.prepare( "INSERT INTO imgTable (filename, imagedata) VALUES ('screenshot.png', :imageData)" );
 +
    query.bindValue( ":imageData", inByteArray );
 +
    if( !query.exec() )
 +
        qDebug() << "Error inserting image into table:\n" << query.lastError();
 +
 
 +
    // Get image data back from database
 +
    if( !query.exec( "SELECT imagedata from imgTable" ))
 +
        qDebug() << "Error getting image from table:\n" << query.lastError();
 +
    query.first();
 +
    QByteArray outByteArray = query.value( 0 ).toByteArray();
 +
    QPixmap outPixmap = QPixmap();
 +
    outPixmap.loadFromData( outByteArray );
 +
    db.close();
 +
 
 +
    // Display image
 +
    QLabel myLabel;
 +
    myLabel.setPixmap( outPixmap );
 +
    myLabel.show();
 +
 
 +
    return app.exec();
 +
}
 +
</source>
  
 
[[Category:Snippets]]
 
[[Category:Snippets]]
 
[[Category:HowTo]]
 
[[Category:HowTo]]

Latest revision as of 14:00, 24 November 2015

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Images (or any other binary data, from files or otherwise) can be stored in a database. One way to do it is converting the data into a QByteArray, and insert it into the database as a Binary Large OBject (BLOB).

Below is a full example of such a workflow, including reading the image back from the database and displaying it:

#include <QtSql>
#include <QtWidgets>

int main( int argc, char *argv[] )
{
    QApplication app( argc, argv );

    // Set up database
    QString dbName( "myDatabase.db3" );
    QFile::remove( dbName ); // delete sqlite file if it exists from a previous run
    QSqlDatabase db = QSqlDatabase::addDatabase( "QSQLITE" );
    db.setDatabaseName( dbName );
    db.open();
    QSqlQuery query = QSqlQuery( db );
    query.exec( "CREATE TABLE IF NOT EXISTS imgTable ( filename TEXT, imagedata BLOB )" );

    // Generate an image (in this case a screenshot) and store it in a QByteArray
    QScreen *screen = app.primaryScreen();
    QPixmap inPixmap = screen->grabWindow( 0 );
    QByteArray inByteArray;
    QBuffer inBuffer( &inByteArray );
    inBuffer.open( QIODevice::WriteOnly );
    inPixmap.save( &inBuffer, "PNG" ); // write inPixmap into inByteArray in PNG format

    // Alternatively, load an image file directly into a QByteArray
    // QFile file("screenshot.png");
    // if (!file.open(QIODevice::ReadOnly)) return;
    // QByteArray inByteArray = file.readAll();

    // Insert image bytes into the database
    // Note: make sure to wrap the :placeholder in parenthesis
    query.prepare( "INSERT INTO imgTable (filename, imagedata) VALUES ('screenshot.png', :imageData)" );
    query.bindValue( ":imageData", inByteArray );
    if( !query.exec() )
        qDebug() << "Error inserting image into table:\n" << query.lastError();

    // Get image data back from database
    if( !query.exec( "SELECT imagedata from imgTable" ))
        qDebug() << "Error getting image from table:\n" << query.lastError();
    query.first();
    QByteArray outByteArray = query.value( 0 ).toByteArray();
    QPixmap outPixmap = QPixmap();
    outPixmap.loadFromData( outByteArray );
    db.close();

    // Display image
    QLabel myLabel;
    myLabel.setPixmap( outPixmap );
    myLabel.show();

    return app.exec();
}