Question

我有一个查询，我已经处理了一个项目，然后我擦除了数据集并重新开始，现在我根本无法获取任何数据。
查询基本上是：

SELECT * 
FROM   TABLEA 
       LEFT JOIN TABLEB 
              ON TABLEA.ID = TABLEB.ID 
WHERE  TABLEA.ID = 1 
       AND TABLEB.DATE = (SELECT MAX(TABLEB.DATE) 
                          FROM   TABLEB 
                          WHERE  TABLEA.ID = 1)

TableB跟踪更改，每个ID有数百个条目。我想要一个与该ID有关的最后一个按时间顺序的项目的单行如果有更好的方法来做到这一点然后真棒，但我真的很想知道为什么这个特定的查询不起作用。当我运行此查询时：

SELECT MAX(TABLEB.DATE) 
FROM   TABLEB 
       LEFT JOIN TABLEB 
              ON TABLEA.ID = TABLEB.ID 
WHERE  TABLEA.ID = 1

我得到数据集中最后一个日期的正确值。

Answer 1

select * 
from tableA as a
    left outer join tableB as b on b.ID = a.ID
where
    b.Date = (select max(t.Date) from tableB as t WHERE t.ID = a.id)
    -- and a.ID = 1 if you need it

如果您只需要来自tableB的日期，则可以

select * 
from tableA as a
    left outer join (
        select max(t.Date) as Date, t.ID from tableB as t group by t.ID
    ) as b on b.ID = a.ID
-- where a.ID = 1 if you need it

如果可以使用row_number函数（可以将公用表表达式更改为子查询）：

with cte as (
    select *, row_number() over(partition by a.ID order by b.Date desc) as rn
    from tableA as a
        left outer join tableB as b on b.ID = a.ID        
    -- where a.ID = 1 if you need it
)
select *
from cte
where rn = 1

如果您使用的是SQL Server版本＆gt; = 2005：

select * 
from tableA as a
    outer apply (
        select top 1 t.*
        from tableB as t
        where t.ID = a.ID
        order by t.Date desc
    ) as b
-- where a.ID = 1 if you need it

请注意在所有子查询中使用别名。

关于您的初始查询，我认为您有错字：

SELECT * 
FROM   TABLEA 
       LEFT JOIN TABLEB 
              ON TABLEA.ID = TABLEB.ID 
WHERE  TABLEA.ID = 1 
       AND TABLEB.DATE = (SELECT MAX(TABLEB.DATE) 
                          FROM   TABLEB 
                          WHERE  TABLEA.ID = 1) -- <-- Do you mean TABLEB.ID = 1 ??

Answer 2

这是一种方式：

SELECT *
FROM tableA LEFT JOIN
     tableB
     ON tableA.ID = tableB.ID
WHERE tableA.ID = 1
ORDER BY tableB.Date desc
LIMIT 1;

SQL按日期获取最后一项

2 个答案: