Query ":placeholder" does not work, but "?" does.

[qtnoob420]

i am trying to get stuff from my ui into a databse. so first i do a check if the entry already exists.

1. if it exists i'll do an UPDATE SET WHERE clause.

2. If not i'll do an INSERT INTO VALUES clause.

in both of these cases, the last part (WHERE / VALUES) a :placeholder does not work.

so here the 2 cases with both examples and how i tried them. with the one working and the other one not working.

1:
//(with ?)
q.prepare("UPDATE my_table SET column1 = :val1 WHERE column2 = ?");
q.bindValue(":val1", ui->my_line_edit->text());
q.bindValue(1, ui->my_line_edit_2->text());
q.exec();
//WORKS

//(with :placeholder)
q.prepare("UPDATE my_table SET column1 = :val1 WHERE column2 = :val2");
q.bindValue(":val1", ui->my_line_edit_1->text());
q.bindValue(":val2", ui->my_line_edit_2->text());
q.exec();
//DOES NOT WORKS

2:
//(with ?)
q.prepare("INSERT INTO my_table (column1) VALUES (?)");
q.bindValue(0, ui->my_line_edit->text());
q.exec();
//WORKS

//(with :placeholder)
q.prepare("INSERT INTO my_table (column1) VALUES (:val)");
q.bindValue(":val", ui->my_line_edit->text());
q.exec();
//DOES NOT WORK

I hope i made the point clear.

[KroMignon]

@qtnoob420 said in Query ":placeholder" does not work, but "?" does.:

I hope i got it this time :D

Yes, that's correct.
I think you problem is link to this ticket QTBUG-48471.
It seems that PostgreSQL do not support prepare statements.

[SGaist]

Hi,

Which version of Qt are you using ?
Which database backend ?

[KroMignon]

@qtnoob420 said in Query ":placeholder" does not work, but "?" does.:

in both of these cases, the last part (WHERE / VALUES) a :placeholder does not work.

to complete @SGaist questions: did you check q.prepare() returned value? If returning false than the query could not be prepared (cf documentation)

[qtnoob420]

@SGaist
QT Creator 4.13.1 (Community)

PostgreSQL 9.5.25

[KroMignon]

@qtnoob420 said in Query ":placeholder" does not work, but "?" does.:

QT Creator 4.13.1 (Community)

This is your IDE, but why Qt Kit you are using to build your application (cf. Tools/Options/Kits)?

[qtnoob420]

@KroMignon
Desktop Qt 5.12.10 MingGW 64-Bit.

I hope i got it this time :D

[KroMignon]

@qtnoob420 said in Query ":placeholder" does not work, but "?" does.:

I hope i got it this time :D

Yes, that's correct.
I think you problem is link to this ticket QTBUG-48471.
It seems that PostgreSQL do not support prepare statements.

[qtnoob420]

@KroMignon as soon as i use my example with the :placeholder i get a syntax error returned from my query. but there is no syntax fault. as i said in my post, i just switched the ":placeholder" with a "?" and the it works fine again.

i suppose there is nothing i can do about it...

[KroMignon]

@qtnoob420 said in Query ":placeholder" does not work, but "?" does.:

i suppose there is nothing i can do about it...

As you can see, it is a limitation of PostgreSQL itself.

if(!q.prepare("UPDATE my_table SET column1 = :val1 WHERE column2 = :val2"))
{
   qDebug() << "Prepare statement error:" << q.lastError().text();
}