cannot get max(column) value in sql.

[rahulvishwakarma]

hi to all, I've Centos7.5 as client and centos6.10 as mysql server. i am building small project and here i want to get maximum value from billno ( bill number ). I've tried this code :-


int Sales::setmaxno()
{
QSqlQuery qury;
QString sql= "select max(billno) from tableSale";

query->prepare(sql);
if(query->exec() )
{
if(qury.next())
{
Sales::maxno = query->value(0).toInt();
return Sales::maxno;
}
}
else
{
QMessageBox::information(this, "Sales", "in else " + query->lastError().text() );
}
}


but this shows maxno = 0; always.
i think query "select max(billno) from tableSale"; is not selecting max of column.
how to get solved this problem.

[Lesiok]

First of all : this code should not compile.

[rahulvishwakarma]

you are right this code is not compiling but Why? please explain.

[Lesiok]

Because the return... line is missing before line 19. The compiler certainly gave the appropriate message.

[d_stranz]

Sales::maxno

Is "maxno" a static member variable of the Sales class? Why do you qualify it with the "Sales::" class scope? Look at your C++ textbook if you do not understand this.

[rahulvishwakarma]

yes maxno is static member

[ChrisW67]

int Sales::setmaxno()
{
QSqlQuery qury;
QString sql= "select max(billno) from tableSale";

query->prepare(sql);
if(query->exec() )
{
if(qury.next())
{
Sales::maxno = query->value(0).toInt();
return Sales::maxno;
}
}
else
{
QMessageBox::information(this, "Sales", "in else " + query->lastError().text() );
}
}


The variable "qury" at line 3 and 9 is not related to the undeclared (member?) variable "query" at lines 6, 7, and 11. You prepare() and exec() one QSqlQuery object then attempt to navigate another (invalid) object. If that worked, you would return value from the first QSqlQuery object, which is pointing at an invalid record. Since "qury" is not active the next() call will fail and your code (if it compiles at all) will return an undefined value because there is no other return statement.