Query

• query所有student表中的資訊

@GetMapping("/students")  
public List<Student> select() {  
    String sql = "SELECT id, name FROM student";  
  
    Map<String, Object> map = new HashMap<>();  
    //可以用lambda表達式傳入rowmapper  
    /*List<Student> studentList = namedParameterJdbcTemplate.query(sql, map, (rs,rowNum) ->  
            new Student(rs.getInt("id"),rs.getString("name"))    );*/    //也可以另外單獨寫一個RowMapper 要繼承RowMapper接口  
    List<Student> studentList = namedParameterJdbcTemplate.query(sql, map, new StudentRowMapper());  
    return studentList;  
}

• 也可以query單獨特定的資訊

@GetMapping("/students/{studentId}")  
public Student select(@PathVariable Integer studentId) {  
    String sql = "SELECT id, name FROM student WHERE id= :studentId";  
    Map<String, Object> map = new HashMap<>();  
    map.put("studentId", studentId);  
  
    List<Student> students = namedParameterJdbcTemplate.query(sql, map,new StudentRowMapper());  
    return students.isEmpty() ? null : students.get(0);  
}

• Rowmapper同時也可以替換成ResultSetExtractor
  • RowMapper一次是一行行讀取
  • ResultSetExtractor是一次性的把所有數據讀出來
  • 小資料集影響不大，大資料集ResultSetExtractor就會有性能疑慮
    

//用resultSetExtractor改寫  
List<Student> students = namedParameterJdbcTemplate.query(sql, map, new ResultSetExtractor<List<Student>>() {  
    @Override  
    public List<Student> extractData(ResultSet rs) throws SQLException, DataAccessException 
    {  
        List<Student> result = new ArrayList<>();  
        while (rs.next()) 
        {  
            Student s = new Student(rs.getInt("id"), rs.getString("name"));  
            result.add(s);  
        }        return result;  
    }
    });  
return students.isEmpty() ? null : students.get(0);