Question

因此，假设您有一个查询来检索数据库中所有棒球运动员的统计信息，该数据会不断更新诸如散步，全垒打等统计信息，例如：

SELECT pl.player_name, pl.player_number, bat.at_bats, pit.era 
FROM players pl
LEFT JOIN batting bat ON pl.player_id = bat.player_id
LEFT JOIN pitching pit ON pl.player_id = pit.player_id

现在说您还想从另一个表（例如薪水表）中提取统计信息，但是您只想提取最近的薪水，所以类似：

SELECT pl.player_name, pl.player_number, bat.at_bats, pit.era, sal.salary
FROM players pl
LEFT JOIN batting bat ON pl.player_id = bat.player_id
LEFT JOIN pitching pit ON pl.player_id = pit.player_id
LEFT JOIN salaries sal ON pl.player_id = sal.player_id

但是您只想获取每个球员的最新工资数字。您将如何结合所有这些？希望我能清楚地解释。

Answer 1

SELECT pl.player_name, pl.player_number, bat.at_bats, pit.era, s.salary
FROM players pl
LEFT JOIN batting bat ON pl.player_id = bat.player_id
LEFT JOIN pitching pit ON pl.player_id = pit.player_id
cross apply (select top 1 * from salaries sal where pl.player_id = sal.player_id order by 
              sal.SalaryDate desc) s

Answer 2

我在本文档中遇到了很多有趣的代码（第6页-它是为SAS编写的，但是由于SAS使用ANSI SQL，所以该代码适用于所有SQL系统）：https://support.sas.com/resources/papers/proceedings17/0930-2017.pdf

对于您的应用程序，它看起来像这样（不是肯定的，但是应该很接近）：

SELECT pl.player_name, pl.player_number, bat.at_bats, pit.era, sal.salary
FROM players pl
LEFT JOIN batting bat ON pl.player_id = bat.player_id
LEFT JOIN pitching pit ON pl.player_id = pit.player_id
LEFT JOIN (
    select salary,
        player_id
    from salaries sal1
    where salDate = (
                    select max(salDate)
                    from salaries sal2
                    where sal1.player_id = sal2.player_id
                   )
           ) as sal ON pl.player_id = sal.player_id

在这种情况下有点痛苦，因为您需要2个子查询来进行描述。我可能会把所有薪水都加入CTE，然后做max（）自加入来选择最近的一个作为最终选择。

多表选择，限制结果数

2 个答案: