如何编写包含不在 GROUP BY 子句中的列的标准 SQL GROUP BY

Question

假设我有一个名为 Customer 的表，定义如下：

Id       Name       DepartmentId    Hired
1        X          101             2001/01/01
2        Y          102             2002/01/01
3        Z          102             2003/01/01

我想检索每个部门最后一次招聘的日期。

显然我会这样做

SELECT c.DepartmentId, MAX(c.Hired)
  FROM Customer c
 GROUP BY c.DepartmentId

返回：

101      2001/01/01
102      2003/01/01

但是，如果我想返回所雇用人员的姓名，我该怎么办？ IE。我想要这个结果集：

101      2001/01/01       X
102      2003/01/01       Z

请注意，以下内容不起作用，因为它将返回三行而不是我要查找的两行：

SELECT c.DepartmentId, c.Name, MAX(c.Hired)
  FROM Customer c
 GROUP BY c.DepartmentId

我不记得看到过实现此目的的查询。

注意：加入 Hired 字段是不可接受的，因为不能保证准确。

Answer 1

子选择将完成这项工作，并处理同一部门在同一天雇用了多个人的情况：

SELECT c.DepartmentId, c.Name, c.Hired from Customer c,
(SELECT DepartmentId, MAX(Hired) as MaxHired
  FROM Customer
 GROUP BY DepartmentId) as sub
WHERE c.DepartmentId = sub.DepartmentId AND c.Hired = sub.MaxHired

Answer 2

标准 Sql：

select * 
from Customer C

where exists
(
  -- Linq to Sql put NULL instead ;-) 
  -- In fact, you can even put 1/0 here and would not cause division by zero error
  -- An RDBMS do not parse the select clause of correlated subquery
  SELECT NULL 

  FROM Customer 

  where c.DepartmentId = DepartmentId

  GROUP BY DepartmentId

  having c.Hired = MAX(Hired)
) 

如果 Sql Server 恰好支持元组测试，这是最简洁的：

select * 
from Customer 
where  (DepartmentId, Hired) in 

(select DepartmentId, MAX(Hired)
 from Customer 
 group by DepartmentId)

Answer 3

SELECT a.*
FROM Customer AS a
JOIN
(SELECT DepartmentId, MAX(Hired) AS Hired
FROM Customer GROUP BY DepartmentId) AS b
USING (DepartmentId,Hired);