具有左连接，其中涉及第二个表中的重复项-MYSQL

Question

表1：

user  score  
------------
A      1    
B      2    

表2：

    user   comment    time
    ----------------------------
    A      good     <timestamp 1>
    A      bad      <timestamp 2>
    B      average  <timestamp 3>

我想加入这两个表，以便获得以下内容：

    user   score  comment
    -------------------------
    A      1        good
    B      2       average

您可以看到，我需要根据时间戳（最近的时间戳）加入第二张表的注释。我尝试过

SELECT st.user as user,st.score,
case when v.comment is null then 'NA' else v.comment end as comment
FROM tale1
left JOIN (select distinct user,comment,max(time) from table2) v ON st.user=v.user

但是这不起作用。

Answer 1

您可以加入相关的子查询以对最新的时间戳进行过滤：

select 
    t1.*,
    t2.comment
from table1 t1
left join table2 t2
    on t2.user = t1.user
    and t2.time = (
        select max(t22.time)
        from table2 t22
        where t21.user = t1.user
    )

Answer 2

您只想要table2中的一列，所以我建议一个相关的子查询：

select t1.*,
       (select t2.comment
        from table2 t2
        where t2.user = t1.user
        order by t2.time desc
        limit 1
       ) as comment
from table1 t1;