Mysql子查询如何仅从Table1中获取所有记录，并且如果找不到与Table2匹配的记录，则仍将其列为null或其他内 - Mysql subquery how can i get only all records from Table1 and if no match found with Table2 still list them as null or something else?

Mysql subquery how can i get only all records from Table1 and if no match found with Table2 still list them as null or something else?

1）我有两个表：

+------                      +---------
topic                         mytask
+---------                   +---------
ID | What                    ID | TopicID
----------                   ------------
1  | A                        1 | 1
2  | B

2）我有如下SQL：

SELECT 
  a.id, b.id
FROM 
  topic a
JOIN (
 SELECT 
  b.id,b.topicid
 FROM 
  mytask b
) b on (b.topicid=a.id)

3）我的输出如下（仅1行）：

ID | ID
-------
1  | 1

我的预期输出如下（2行，通过列出所有主题优先）：

a.ID | b.ID
-------------
1    | MATCHED - OK - TAKE IT!!!
2    | NULL or what,ever...

我该怎么做？

使用左外联接：

SELECT 
  a.id, b.id
FROM 
  topic a
LEFT JOIN (
 SELECT 
  b.id,b.topicid
 FROM 
  mytask b
) b on (b.topicid=a.id)

并将查询简化为

SELECT 
  a.id, b.id
FROM 
  topic a
LEFT JOIN 
  mytask b ON b.topicid = a.id