query.next() returning false - sql

Question

I have used the query.next() function inside my code, but its returning false.
I even checked in the database, there are 4 records present. But the code shows only one.
However if i use query.next() before the code of query.valid() then it doesn't show any record
Please help
qDebug() << "entering payment: get all the unchecked invoices for user: " + user;
QStringList tmp;
QSqlQuery query(m_storageUserManager->database());
m_storageUserManager->dumpTable(m_invoiceInfoTable);
m_storageUserManager->dumpTable(m_invoiceUserTable);
qDebug()<<"THE NAME OF THE INVOICE USER TABLE_----=-----------------"<<m_invoiceInfoTable;
qDebug()<<"THE NAME OF THE INVOICE USER TABLE_----=-----------------"<<m_invoiceUserTable;
query.prepare("SELECT invoice FROM "+ m_invoiceInfoTable +" WHERE invoice = (SELECT
invoice FROM "+ m_invoiceUserTable +" WHERE user=:user)");
// query.prepare("SELECT invoice FROM " + m_invoiceInfoTable + ","+ m_invoiceUserTable +" WHERE " + m_invoiceInfoTable + ".user = " + m_invoiceUserTable + ".:user");
query.bindValue(":user", user);
query.exec();
query.first();
qDebug()<<"Unchecked invoices done!!! " ;
if(query.isValid()) {
do {
tmp.append(query.value(0).toString()); //as the value returned by value() is a QVariant so we need to change it to String.
} while(query.next());
} else
tmp.append("No Unchecked invoice in the database");
return tmp;

Answer 1

To check if the query was successful you should test the return value of either QSqlQuery::exec() or QSqlQuery::isActive() before trying to call first/next/last (when you pass the query string to the constructor of QSqlQuery, the query is already executed, so, you need to use QSqlQuery::isActive()).
first(), next() and last() return true if they positioned the query on a valid record, you don't have to test isValid() separately. Since first() is a positioning function too, you can read the value without calling next() directly after, unless you want to skip the first record.
Since you may want to add fields from the the "invoice-info" table to your query, I kept the subquery (with IN instead of = as Mat already answered in the comment).
query.prepare(QString("SELECT invoice FROM %1 WHERE invoice "
"IN (SELECT invoice FROM %2 WHERE user=:user)")
.arg(m_invoiceInfoTable, m_invoiceUserTable));
/*
// Or with an inner join
query.prepare(QString("SELECT %1.invoice FROM %1 "
"INNER JOIN %2 ON %1.invoice = %2.invoice "
"WHERE user=:user").arg(m_invoiceInfoTable, m_invoiceUserTable));*/
query.bindValue(":user", user);
if (!query.exec()) {
tmp.append("Query error: %1" + query.lastError().text());
} else if (!query.first()) {
tmp.append("No Unchecked invoice in the database");
} else {
do {
tmp.append(query.value(0).toString());
} while(query.next());
}

Answer 2

Try
query.prepare("SELECT invoice FROM " + m_invoiceInfoTable + " WHERE user=:user");