class DBFunctions
{
...
private:
    QString squery;
    QSqlQuery *q;
};
QString DBFunctions::prepareQueries()
{
    q = new QSqlQuery();
    squery = "INSERT INTO T_BLOCK (A, B, C) VALUES (:A, :B, :C)";
    if (!q->prepare(squery))
        return "QUERY_PREP_ERR: " + QString("'%1' %2")
          .arg("INSERT INTO T_BLOCK").arg(q->lastError().text());
    return "";
}
QString DBFunctions::insertToDB(const int x)
{
    for (int i = 1; i <= 3; i++) {
        q->bindValue(":A", 100 * i + x); //тестовый вариант
        q->bindValue(":B", 100 * i + x);
        q->bindValue(":C", 100 * i + x);
        q->exec();
        if (!q->isActive())
            return "QUERY_EXEC_ERR: " + QString("'%1' %2")
              .arg("INSERT INTO T_BLOCK").arg(q->lastError().text());
    }
    return "";
}

QString DBFunctions::insertToDB()
{
    QString squery;
    int qntRec = 10;

    QSqlDatabase::database().transaction();
    QSqlQuery q;
    squery = "INSERT INTO T_BLOCK (A, B, C) VALUES (:A, :B, :C)";
    if (!q.prepare(squery)) {
        QSqlDatabase::database().rollback();
        return "QUERY_PREP_ERR: " + QString("'%1' %2")
          .arg("INSERT INTO T_BLOCK").arg(q.lastError().text());
    }
    for (int i = 1; i <= qntRec; i++) {
        q.bindValue(":A", i + 1); //тестовый вариант
        q.bindValue(":B", i + 2);
        q.bindValue(":C", i + 3);
        q.exec();
        if (!q.isActive()) {
            QSqlDatabase::database().rollback();
            return "QUERY_EXEC_ERR: " + QString("'%1' %2")
              .arg("INSERT INTO T_BLOCK").arg(q.lastError().text());
        }         
    }         
    QSqlDatabase::database().commit();

    return "";
}

QString squery;
int ID = 5;
QSqlQuery q;
squery = "SELECT FIRST 1 ID FROM T1 WHERE ID = " + QString::number(ID);
q.exec(squery);
int N = (q.next()) ? 10 : 20;
QSqlDatabase::database().transaction();
squery = "DELETE FROM T2 WHERE ID = " + QString::number(ID);
q.exec(squery);
if (!q.isActive()) {
    QSqlDatabase::database().rollback();
    return;
}
if (!q.prepare("INSERT INTO T2 (ID, N, F) VALUES (:ID, :N, :F)")) {
    QSqlDatabase::database().rollback();
    return;
}
q.bindValue(":ID", ID);
q.bindValue(":N", N);
for (int i = 0; i <= 10; i++)
{
    q.bindValue(":F", i);
    q.exec();
    if (!q.isActive()) {
        QSqlDatabase::database().rollback();
        return;
    }
}
QSqlDatabase::database().commit();

    QString squery;
    int ID = 10;

    QSqlDatabase::database().transaction();
    QSqlQuery q;
    squery = "DELETE FROM T_BLOCK “
             ”WHERE ID > " + QString::number(ID);
    q.exec(squery);
    if (!q.isActive()) {
        QSqlDatabase::database().rollback();
        return "QUERY_EXEC_ERR: " + QString("'%1' %2")
          .arg("DELETE FROM T_BLOCK").arg(q.lastError().text());
    }
    squery = "SELECT COUNT(*) FROM T_BLOCK “
             ”WHERE ID > " + QString::number(ID);
    q.exec(squery);
    if (!q.isActive()) {
        QSqlDatabase::database().rollback();
        return "QUERY_EXEC_ERR: " + QString("'%1' %2")
          .arg("SELECT COUNT(*) FROM T_BLOCK").arg(q.lastError().text());
    }
    QSqlDatabase::database().commit();

    return "";

QString squery;
int ID = 10;

QSqlDatabase::database().transaction();
QSqlQuery q;
squery = "SELECT * FROM T1";
q.setForwardOnly(true); //перед exec неподготавливаемого запроса
q.exec(squery);
while (q.next()) {
    …
}
squery = "SELECT * FROM T2 WHERE ID > :ID";
q.setForwardOnly(true); //перед prepare подготавливаемого запроса
q.prepare(squery);
q.bindValue(":ID", ID);
q.exec();
while (q.next()) {
    …
}
QSqlDatabase::database().commit();