Column must appear in the GROUP BY clause or be used in an aggregate function - postgresql

Question

I'm updating a Qt software, to make it compatible with both SQLite and PostgreSQL.
I have a C++ method that is used to count elements of a given table with given clauses.
In SQLite, the following worked and gave me a number N (the count).
SELECT COUNT(*) FROM table_a
INNER JOIN table_b AS
ON table_b.fk_table_a = table_a.id
WHERE table_a.start_date_time <> 0
ORDER BY table_a.creation_date_time DESC
With PostgreSQL (I'm using 9.3), I have the following error :
ERROR: column "table_a.creation_date_time" must appear in the
GROUP BY clause or be used in an aggregate function
LINE 5: ORDER BY
table_a.creation_date_time DESC
If I add, GROUP BY table_a.creation_date_time, it gives me a table with N rows.
I've read a lot of stuff about how different DBMS allow you to omit columns in the GROUP BY clause. Now, I'm just confused.
For those who are curious, the C++ method is:
static int count(const QString &table, const QString &clauses = QString(""))
{
int success = -1;
if (!table.isEmpty())
{
QString statement = QString("SELECT COUNT(*) FROM ");
statement.append(table);
if (!clauses.isEmpty())
{
statement.append(" ").append(clauses) ;
}
QSqlQuery query;
if(!query.exec(statement))
{
qWarning() << query.lastError();
qWarning() << statement;
}
else
{
if (query.isActive() && query.isSelect() && query.first())
{
bool ok = false;
success = query.value(0).toInt(&ok);
if (ok == false)
{
success = -1;
return success;
}
}
}
}
return success;
}

Answer 1

If you're just doing a count(*) on the table in order to get a single scalar-value result, then surely having the order by present is obsolete ?
solution
Remove the obsolete order by to get "standard" query behavior across multiple dbms