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Thread: cannot get max(column) value in sql.

  1. #1
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    Default cannot get max(column) value in sql.

    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 :-
    Qt Code:
    1. int Sales::setmaxno()
    2. {
    3. QSqlQuery qury;
    4. QString sql= "select max(billno) from tableSale";
    5.  
    6. query->prepare(sql);
    7. if(query->exec() )
    8. {
    9. if(qury.next())
    10. {
    11. Sales::maxno = query->value(0).toInt();
    12. return Sales::maxno;
    13. }
    14. }
    15. else
    16. {
    17. QMessageBox::information(this, "Sales", "in else " + query->lastError().text() );
    18. }
    19. }
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    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.

  2. #2
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    Default Re: cannot get max(column) value in sql.

    First of all : this code should not compile.

  3. #3
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    Default Re: cannot get max(column) value in sql.

    you are right this code is not compiling but Why? please explain.
    Last edited by rahulvishwakarma; 1st May 2020 at 16:05.

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    Default Re: cannot get max(column) value in sql.

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

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    Default Re: cannot get max(column) value in sql.

    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.
    <=== The Great Pumpkin says ===>
    Please use CODE tags when posting source code so it is more readable. Click "Go Advanced" and then the "#" icon to insert the tags. Paste your code between them.

  6. #6
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    Default Re: cannot get max(column) value in sql.

    yes maxno is static member

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    Default Re: cannot get max(column) value in sql.

    Qt Code:
    1. int Sales::setmaxno()
    2. {
    3. QSqlQuery qury;
    4. QString sql= "select max(billno) from tableSale";
    5.  
    6. query->prepare(sql);
    7. if(query->exec() )
    8. {
    9. if(qury.next())
    10. {
    11. Sales::maxno = query->value(0).toInt();
    12. return Sales::maxno;
    13. }
    14. }
    15. else
    16. {
    17. QMessageBox::information(this, "Sales", "in else " + query->lastError().text() );
    18. }
    19. }
    To copy to clipboard, switch view to plain text mode 

    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.

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