How to get value from a query statement ?

[Lykurg]

Hi, since you only query one "field" the index is 0. Thus you can simplify your code:

QSqlQuery q("select count(MaDaiLy) from DaiLy, Quan where DaiLy.Quan_TenQuan = Quan.TenQuan and Quan.TenQuan = '" + ui.cbTenQuan->currentText() + "'");
q.next();
int SoDLHienCoTrongQuan = q.value(0).toInt();

QSqlQuery q("select count(MaDaiLy) from DaiLy, Quan where DaiLy.Quan_TenQuan = Quan.TenQuan and Quan.TenQuan = '" + ui.cbTenQuan->currentText() + "'");
q.next();
int SoDLHienCoTrongQuan = q.value(0).toInt();

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Further have a look at QSqlQuery::bindValue().