PHP+MySQL查询指定字段时无法获取tbl_class表数据问题

• 未在SELECT子句中声明你需要获取的tbl_class关联表字段：你仅指定了tbl_child表的字段，LEFT JOIN得到的tbl_class数据不会自动被返回
• 存在SQL语法错误：child_age字段末尾多了多余的逗号，会导致SQL语句解析失败
• 未给字段加表别名前缀，若两表存在同名字段会触发字段歧义错误

$select = $pdo->prepare(" select 
                   c.childpid,
                   c.child_name,
                   c.child_surname,
                   c.child_birth,
                   c.child_age,
                   -- 下面是tbl_class表字段，按需保留即可
                   l.classpid,
                   l.class_courseyear,
                   l.class_course
                   from tbl_child c 
                   
                   LEFT JOIN (SELECT classpid,childfkey,class_courseyear,class_course FROM tbl_class  where classpid IN (
                    
                    SELECT MAX(classpid)
                    FROM tbl_class 
                    GROUP BY childfkey
                    
                    
                   )) l ON l.childfkey=c.childpid");

内容的提问来源于stack exchange，提问作者anubs