QSqlQuery with back slash - c++

Question

I'm trying to retrieve the users from the database their names are 'DOMAIN\name'.
I've checked the query in sql console, simple select like :
select * from users where name='DOMAIN\\name'
it returns correct row if the name in the database looks like 'DOMAIN\user' (single back slash).
however QSqlQuery returns empty :
The code something like:
const QString command = QStringLiteral("select * "
"from %1 where name = '%2'")
.arg(Constants::kUsersTableName).arg(userId);
qCDebug() << "Query:" << command;
QSqlDatabase db = QSqlDatabase::database(m_connection, false);
QSqlQuery query(db);
if (!query.prepare(command) || !query.exec()) {
...
log :
Query: "select * from users where name = 'DOMAIN\\name'"
any idea why QSqlQuery returns empty at the same time when database console return valid record for the user.
dbms: MySQL

Answer 1

MySQL uses \ as its escape character therefore the correct query string you have to escape the slash twice, once for c++ and once for mysql:
"select * from users where name='DOMAIN\\\\name'"
The simpler solution is to use prepared statements and placeholders correctly which as a very important bonus will protect your code from SQLI.
Note that field and table names aren't allowed to be placeholders by most if not all database engines so you will still need to build that part of your string by hand:
const QString usernamePlaceholder = ":username"
const QString command = QStringLiteral("select * "
"from %1 where name = %2")
.arg(Constants::kUsersTableName).arg(usernamePlaceholder);
QSqlDatabase db = QSqlDatabase::database(m_connection, false);
QSqlQuery query(db);
if (!query.prepare(command)) {
qCDebug() << query.lastError();
return;
}
query.bindValue(usernamePlaceholder, userId);
if (!query.exec()) {
qCDebug() << query.lastError();
return;
}
bindValue will take care of whatever quoting and escaping is required for your values.