如何在Java中正确地从数据库检索数据

Question

I wrote this code in order to display the name of employee who has the maximum salary but when the output wasn't correct it appeared null not "mmm kkk" !! although I filled the table and this is the contents :

HERE

this is my code, can any one help me ?? :(

 public static void displayMaxSalary() throws ClassNotFoundException, SQLException

{   
    int size=0;
    int count=0;
    String maxSalary=null;
    Class.forName("com.mysql.jdbc.Driver");
    Connection con = DriverManager.getConnection("jdbc:mysql://localhost/task4DB?"
              + "user=root&password=123");
    PreparedStatement st = con.prepareStatement("select * from task4Table ");
    ResultSet r1=st.executeQuery();

      while (r1.next()) {

          size++;
      }


      int salaries[]=new int[size];
      String Names[]=new String[size];

       while (r1.next()) {


          salaries[count]= r1.getInt("salary");
          Names[count]=  r1.getString("fName")+""+r1.getString("lName");
          count++;
      }

       for(int i=1;i< salaries.length;i++)

       {
           if(salaries[i]>salaries[i-1])
           {
               maxSalary= Names[i];
           }

       }


     System.out.println("The name of employee who has the higher salary is :");
     System.out.println( maxSalary);



} //end-displayMaxSalary.

Answer 1

Try this SQL statement:

select fName, max(salary) from task4table

---

A problem with your code:

In this loop you iterate to the end of the result set:

while (r1.next()) {
    size++;
}

And then, when you want to iterate again

while (r1.next()) {
    salaries[count]= r1.getInt("salary");
    Names[count]=  r1.getString("fName")+""+r1.getString("lName");
    count++;
}

r1.next() returns false so this iteration does not happen. You either do it in one loop, or you execute query again. But, as I said, to it with proper sql statement using max .

Answer 2

You change query to get all maximum salary persons in single query

PreparedStatement st = con.prepareStatement("
select fName, lName, salary from task4Table where salary = 
(select max(salary) from task4Table)");
ResultSet r1 = st.executeQuery();

Iterate this resultset.

Answer 3

There's a lot wrong with your code, but here's your immediate problem: you aren't finding the max correctly.

The right way is to use SQL, as the answer above mine shows. If you insist on going your way, try this:

int maxSalary = Integer.MIN_VALUE;
for (int i = 0; i < salaries.length; i++) {
    if (salaries[i] > maxSalary) {
        maxSalary = salaries[i];
    }
}

Your coding style is not good. You need to be consistent about brace placement, spaces, blank lines, etc. You should also memorize and follow the Sun Java coding conventions.

Answer 4

Try following code:

public static void displayMaxSalary() throws ClassNotFoundException, SQLException

{   

    String maxSalary;
    Class.forName("com.mysql.jdbc.Driver");
    Connection con = DriverManager.getConnection("jdbc:mysql://localhost/task4DB?"
              + "user=root&password=123");
    PreparedStatement st = con.prepareStatement("select * from task4Table order by salary desc limit 1");
    ResultSet r1=st.executeQuery();

     if(r1.next()) {

           maxSalary =  r1.getString("fName")+""+r1.getString("lName");
      }

     System.out.println("The name of employee who has the higher salary is :");
     System.out.println( maxSalary);


} //end-displayMaxSalary.