Java中的一对多ResultSet

Question

My employee data is:-

NAME | ID  | DEP_CODE
 AA   3679  SALES
 AA   3679  HR
 AA   3679  TECH
 AA   3679  MARKETTING
 BB   3604  HR
 BB   3604  SALES

Can someone revise it and help me to remove the need of FOR loop here and fetch list of employee in one go? I want to make DB call only once, I want to fetch DEP_CODE based on ID.

public List<Employee> fetchDistinctEmployee() {

    List<Employee> empList = new ArrayList<Employee>();
        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery("select distinct NAME, ID FROM Emp_Table");

        while (rs.next()) {
            Employee ep = new Employee();
            em.setNAME(rs.getString("NAME"));
            em.setID(rs.getInt("ID"));
            empList.add(em);
        }
        for (Employee emp : empList) {
            String sql1 = "SELECT DISTINCT DEP_CODE FROM Emp_Table WHERE id='" + emp.getID() + "'";
            ResultSet rs1 = stmt.executeQuery(sql1);
            ArrayList<String> list = new ArrayList<String>();
            while (rs1.next()) {
                list.add(rs1.getString("DEP_CODE"));
            }
            emp.setDEPCODE(list);
        }
    return empList;
}           

Answer 1

I would use a Map:

    Map<Integer,Employee> empMap = new LinkedHashMap<>();
    try (Statement stmt = conn.createStatement()) {
        ResultSet rs = stmt.executeQuery("select distinct NAME, ID, DEP_CODE FROM Emp_Table");

        while (rs.next()) {
            int id = rs.getInt("ID");
            Employee em = empMap.get(id);
            if (em == null) {
                em = new Employee();
                em.setID(id);
                em.setNAME(rs.getString("NAME"));
                em.setDEPCODE(new ArrayList<>()); // could be done in the constructor of Employee instead
                empMap.put(id, em);
            }
            em.addDEPCODE(rs.getString("DEP_CODE"));
        }
    }
    return new ArrayList<>(empMap.values());

Method addSEPCODE would add one value to the SEPCODE list.

Note BTW that it's a good idea to close a statement when finished with it.

Answer 2

The code below is only a piece of advice, I hope it is helpful. Remember these code haven't been tested.

public List<Employee> fetchDistinctEmployee() {
    Map<String,Employee> empMap = new HashMap<String,Employee>();

    Statement stmt = conn.createStatement();
    ResultSet rs = stmt.executeQuery("select NAME, ID, DEP_CODE FROM Emp_Table");

    while (rs.next()) {
        String id = rs.getInt("ID");
        Employee ep = empMap.get(id);
        if(ep==null){
            ep = new Employee();
            empMap.put(id, ep);
            ep.setID(id);
            ep.setNAME(rs.getString("NAME"));
        }
        ep.getDEPCODE.add(rs.getString("DEP_CODE"));
    }
    return new ArrayList<Employee>(empMap.values());
}    

Answer 3

Just modifying OP's code and looping only once:

public List<Employee> fetchDistinctEmployee() {

        List<Employee> empList = new ArrayList<Employee>();
        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery("select NAME, ID, DEP_CODE FROM Emp_Table sort by ID");

        Employee em = null ;
        while (rs.next()) {
            if(em == null || em.getID() != rs.getInt("ID")) {
                em = new Employee();
                em.setDEPCODE(new ArrayList<String>());
                empList.add(em);
            }
            em.setNAME(rs.getString("NAME"));
            em.setID(rs.getInt("ID"));
            em.getDEPCODE().add(rs.getString("DEP_CODE"));
        }
    return empList;
}