QSqlQuery weird error

[krzysieklfc]

In the following code, q.prepare() and q.exec() both return false and qDebug outputs - " Parameter count mismatch"

void Database::deleteAllRecords(const QString& db_name, const QString& table)
{
	auto err = initDb("deleteData", db_name);
	Q_ASSERT(err.type() == QSqlError::NoError);

	auto db = QSqlDatabase::database("deleteData");

	auto q = QSqlQuery{ db };
	Q_ASSERT(q.driver()->hasFeature(QSqlDriver::NamedPlaceholders));

	auto zxda = q.prepare(QLatin1String{ R"(
		DELETE FROM :table
	)" });

	//the same with addBindValue
	q.bindValue(":table", table);

	auto ok = q.exec();

	qDebug() << q.lastError().text();
}

but when I supply hardcoded table name then it all works just fine:

void Database::deleteAllRecords(const QString& db_name)
{
	auto err = initDb("deleteData", db_name);
	Q_ASSERT(err.type() == QSqlError::NoError);

	auto db = QSqlDatabase::database("deleteData");

	auto q = QSqlQuery{ db };
	Q_ASSERT(q.driver()->hasFeature(QSqlDriver::NamedPlaceholders));

	auto zxda = q.prepare(QLatin1String{ R"(
		DELETE FROM cache
	)" });

	auto ok = q.exec();

	qDebug() << q.lastError().text();
}

Db driver is QSQLITE. Does anyone have an idea what's going on there?

[JonB]

@krzysieklfc
I could be off, because I've never used SQLite, but you can only put placeholder-bind-variables where the SQL actually allows it, it's not just "text-like-macro-substitution". I think you will find DELETE FROM <table-name> does not allow a "bound variable" there, it has to be literal, hence the behaviour you see. Essentially I think you can use the bound-variables for column values & literals, but not e.g. for a table or database name etc.

[krzysieklfc]

Thanks for reply.

I managed to solve this problem with the following:

	auto prepare_string = QString{ "DELETE FROM %1" }.arg(table);
	auto ok = q.prepare(prepare_string);

	auto ok = q.exec();