QT执行MYSQL语句问题解决_qsqlquery::exec: database not open can't open data

int ServerBase::DBConnection()
{
    m_sqldb = QSqlDatabase::addDatabase(DB_SALES_DRIVER);
    m_sqldb.setHostName(DB_SALES_HOST);
    m_sqldb.setDatabaseName(DB_SALES_DBNAME);
    m_sqldb.setUserName(DB_SALES_USER);
    m_sqldb.setPassword(DB_SALES_PASSWD);
    if( !m_sqldb.open())
    {
        m_dbconnected = false;
        qDebug()<<" [Error] "<<"Database open error";//<<m_sqlquery.lastError();
        return R_ERROR;
    }
    else
    {
        m_dbconnected = true;
        qDebug()<<" [OK] "<<"Database opened";
    }
    return R_OK;
}
 
    int result = this->DBConnection();
    m_sqlquery = new QSqlQuery("",m_sqldb);//statement 1:connect db with sqlquery
    if(result == R_OK)
    {
        result = m_sqlquery->exec("INSERT INTO children(fname,age) VALUES('Ann2',13)");
        if(!result)
           qDebug()<<" [OK] "<<"EXEC successed";
        m_sqlquery->exec("SELECT * FROM children c LIMIT 0,1000");
        while(m_sqlquery->next())
        {
            qDebug()<<m_sqlquery->value(0).toString()<<m_sqlquery->value(1).toString();
        }
    }
 
//Debug输出：
 [OK]  Database opened 
"1" "Ann" 
"2" "Ann" 
"3" "Ann2" 
"4" "Ann2" 
 [OK]  Database closed 

m_sqldb = QSqlDatabase::addDatabase(DB_SALES_DRIVER);
    m_sqldb.setHostName(DB_SALES_HOST);
    m_sqldb.setDatabaseName(DB_SALES_DBNAME);
    m_sqldb.setUserName(DB_SALES_USER);
    m_sqldb.setPassword(DB_SALES_PASSWD);
    if( !m_sqldb.open())
    {
        m_dbconnected = DB_UNCONNECTED;
        qDebug()<<" [Error] "<<"Database open error";//<<m_sqlquery.lastError();
        return R_ERROR;
    }
    else
    {
        m_sqlquery = new QSqlQuery("",m_sqldb);
        m_dbconnected = DB_CONNECTED;
        qDebug()<<" [OK] "<<"Database opened";
    }