QVariant returns invalid meanwhile the value is valid

Question

I have a small function like this

bool QcgDatabase::onceindb(const QString& userId)
{
    mDb->prepareSqlQuery("SELECT count(*) FROM mytable WHERE userid=:userId;", "database");
    mDb->prepareBindValue(":userId", userId);
    mDb->sqlExec();

    bool d = mDb->sqlQuery().isActive();
    QVariant c = mDb->sqlQuery().value(QString("count(*)"));
    int e = c.toInt();

    if (e == 1) {
        return true;
    }
    else {
        return false;
    }
}

When I do this command in SQL, the result looks like this

I just want to take value 2 for comparison below, but in the code when I debug, QVariant c always return invalid, therefore e is always = 0. I thought my SQL command is not active, but when I debug, bool dalways return true. Do you guys know why? how can I receive 2 as expected ?

@HonestAbe: yeah, sorry, it should be `e==1`. I correct it already, Could you help me to point out problem ? — trangan, Aug 04 '16 at 06:39
@hyde: I checked again with `bool valid = mDb->sqlQuery().isValid();` the result is `false`. But I still don't know why it is invalid. :( — trangan, Aug 04 '16 at 08:38
You should probably read the docs of `mDb`... You want something like http://doc.qt.io/qt-5/qsqldatabase.html#lastError — hyde, Aug 04 '16 at 10:38

score 0 · Answer 1 · answered Aug 05 '16 at 14:16

Your problem is that you're trying to fetch the result from a column with a special (and in some places invalid) name count(*). You can set an alias for the column and fetch the value using that column alias:

...
mDb->prepareSqlQuery("SELECT count(*) as count FROM mytable WHERE userid=:userId;", "database");
...
QVariant c = mDb->sqlQuery().value("count");
...

And checking return values for error states is also good practice...

QVariant returns invalid meanwhile the value is valid

1 Answers1