how to find the maximum value of a column mysql and return the value?

Question

I want to search the maximum value of a column but the value found receives 0 all the time

public  int maXnumR()
{
    DataBase s = DataBase.getInstance();
    int numR= getnumR();

    String req1 = "SELECT max(`idrf`) FROM   `reference`  WHERE `numR` = "
        + numR + " GROUP BY `numR` ";

    try 
    {
        Statement m=  s.getConn().createStatement();
        ResultSet r1 = m.executeQuery(req1);
        while (r1.next()) 
        {
            maxnumR =r1.getInt("idrf");
            nbp++;  
        } 
    } 
    catch (SQLException e1) 
    {
        e1.printStackTrace();
        System.out.println("maXnumR : "+e1);
    } 
    return maxnumR;
}

maXnumR return 0 and the table is not empty.

if I execute the query it works fine with MySQL

erreur:

java.sql.SQLException: Column 'idrf' not found.
maXnumR : java.sql.SQLException: Column 'idrf' not found.
idrf existe:0

Answer 1

尝试

SELECT max(`idrf`) as idrf

Answer 2

Like Luca said, the problem is that your query doesn't return a column named "idrf", but something more like: "max(idrf)".

This means that when you use getInt("idrf") there is no match in the resultSet returned by your query.

That being said, you need to use

SELECT max(`idrf`) as idrf

(or any other name for your resultSet column), like Luca answered, and use that as the key to your getInt.