PreparedStatement，用于读取临时参数

Question

I am trying to read a temporary variable created from my MySql query as follows:

String name = "";
                            selectQuery = "select (select "
                                    + "CONCAT(s.firstname,\" \",s.surname) "
                                    + "AS name) "
                                    + "from student s, marks m where m.grade = 'fail'";
                            try{
                                pstmt = con.prepareStatement(selectQuery);

                                rs = pstmt.executeQuery(selectQuery);
                                int count = 0;

                                while(rs.next()){
                                    name = rs.getString("name");
                                    System.out.println(++count+". "+name);
                                }
                                rs.close();
                                pstmt.close();
                            }catch(Exception e){
                                e.printStackTrace();
                            }

I get SQLException as column 'name' not found. When I run the query in MySql server it runs fine.

Answer 1

In order for the alias to apply, it must be places outside the nested query's parenthesis, not inside it. However, you could just drop it altogether and just use the concat call directly in the select list.

Two more side notes:

1. Implicit joins (ie, placing more than one table in the from list) have been deprecated for quite a while. It's considered a better practice to use explicit joins.
2. Regardless of the joining syntax you're using, you're missing the join condition.
3. Using concat_ws instead of concat may save you some hassle with handling the white space yourself.

To make a long story short:

select CONCAT_WS(' ', s.firstname, s.surname) AS name
FROM   student s
JOIN   marks m ON s.id = m.student_id
WHERE  m.grade = 'fail'