Question

Note: this is with SQLite, although I expect the problem is on the Qt side.

First, I set up a database table from the SQLite command line tool:

sqlite> create table testtable ( id INTEGER PRIMARY KEY NOT NULL, state INTEGER );
sqlite> insert into testtable (state) values (0);
sqlite> insert into testtable (state) values (1);
sqlite> insert into testtable (state) values (9);
sqlite> insert into testtable (state) values (20);

Then I test my query:

sqlite> SELECT id,state FROM testtable WHERE state IN (0,1,2);
1|0
3|1

(Those are expected results.)

Then I run this C++ code:

void runQuery() {
        QSqlQuery qq;
        qq.prepare( "SELECT id,state FROM testtable WHERE state IN (:states)");
        QList<QVariant> statesList = QList<QVariant>();
        statesList.append(0);
        statesList.append(1);
        statesList.append(2);
        qq.bindValue(":states", statesList);
        qq.exec();
        qDebug() << "before";
        while( qq.next() ) {
            qDebug() << qq.value(0).toInt() << qq.value(1).toInt();
        }
        qDebug() << "after";
}

which prints this:

before
after

No rows were printed. I assume this is because I can’t bind a list directly to a placeholder in an “in” clause. But is there a way to do it? I haven’t been able to find anything about this.

Answer 1

Never mind my question. I think what I am trying to do is not possible with prepared statements, regardless of the framework or RDBMS. You can do “WHERE x IN (?)”, but then the ‘?’ refers to an single value — it cannot be a list of values; or you can do “WHERE x IN (?,?,?), and each ‘?’ needs to be bound separately.

Example:

QString const queryText = "SELECT id, firstname FROM users WHERE id IN (%1)";

QVector<int>     const ids { /* ... */ };
QVector<QString> const placeholders(ids.size(), "?");

QSqlQuery query(db);
query.prepare(queryText.arg(QStringList::fromVector(placeholders).join(", ")));

for (auto const & i : ids)
    query.addBindValue(i);

query.exec();