Question

您好我对左连接有疑问。我有两张桌子＆＃34; Customer＆＃34;，＆＃34; Meeting＆＃34;常见栏目是＆＃34; mkod＆＃34;。

客户表有＆＃34; mkod＆＃34;和＆＃34; customername＆＃34; colums 会议桌有＆＃34; mkod＆＃34;，＆＃34;详情＆＃34;和＃34; meetdate＆＃34;

我希望看到客户的所有记录和＆＃34;详细信息＆＃34; mkod上的列和会议表中的最后一次会议日期。

但是我看不到会议的所有记录。

我的代码就是这样，

select c.mkod,m.details 
from customer c 
left join meeting m on c.mkod=m.mkod 
where m.details=
    (select top 1 details 
    from meeting 
    where c.mkod=meeting.mkod 
    order by meetingdate desc) 
order by c.mkod

谢谢

Answer 1

试试这个：

<强>的MySQL

SELECT c.mkod, IFNULL(m.details, '') AS details
FROM customer c 
LEFT JOIN (SELECT m.mkod, m.details
           FROM meeting m 
           INNER JOIN (SELECT m.mkod, MAX(m.meetingdate) meetingdate 
                       FROM meeting m 
                       GROUP BY m.mkod
                     ) a ON m.mkod = a.mkod AND m.meetingdate = a.meetingdate 
         ) m ON c.mkod=m.mkod;

SQL Server

SELECT c.mkod, ISNULL(m.details, '') AS details
FROM customer c 
LEFT JOIN (SELECT m.mkod, m.details
           FROM meeting m 
           INNER JOIN (SELECT m.mkod, MAX(m.meetingdate) meetingdate 
                       FROM meeting m 
                       GROUP BY m.mkod
                     ) a ON m.mkod = a.mkod AND m.meetingdate = a.meetingdate 
         ) m ON c.mkod=m.mkod;

Answer 2

这是您的查询：

select c.mkod, m.details
from customer c left join
     meeting m
     on c.mkod=m.mkod
where m.details=(select top 1 details from meeting where c.mkod=meeting.mkod order by meetingdate desc)
order by c.mkod;

where子句在left outer join的第二个表上有一个条件。这意味着当列值为NULL时，条件不会返回true - 将left outer join转换为inner join。您可以通过将条件移动到on子句来解决此问题：

select c.mkod, m.details
from customer c left join
     meeting m
     on c.mkod = m.mkod and
        m.details = (select top 1 m2.details from meeting m2 where c.mkod = m2.mkod order by m2.meetingdate desc)
order by c.mkod;

此外，如果您想要最新的，那么条件不应该是meetingdate而不是details吗？

select c.mkod, m.details
from customer c left join
     meeting m
     on c.mkod = m.mkod and
        m.meetingdate = (select max(m2.meetingdate) from meeting m2 where c.mkod = m2.mkod)
order by c.mkod;

Sql左连接错误

2 个答案: