T-SQL 获取特定组的值

Question

我有一个类似这样的 EmployeeContract 表：

ContractId	EmployeeId	ValidFrom	ValidTo	Salary
12	5	2018-02-01	2019-06-31	x
25	8	2015-01-01	2099-12-31	x
50	5	2019-07-01	2021-05-31	x
52	6	2011-08-01	2021-12-31	x
72	8	2010-08-01	2014-12-31	x
52	6	2011-08-01	2021-12-31	x

表格包括公司中每位员工的历史合同。我需要知道员工开始工作的日期和合同的最后日期。有时记录有重复。

例如，根据上面的数据：

EmployeeId	ValidFrom	ValidTo
5	2018-02-01	2021-05-31
8	2010-08-01	2099-12-31
6	2011-08-01	2021-12-31

根据这篇文章：https://www.techcoil.com/blog/sql-statement-for-selecting-the-latest-record-in-each-group/

我准备了这样的查询：

select minv.*, maxv.maxvalidto from
    (select distinct con.[EmployeeId], mvt.maxvalidto 
    from [EmployeeContract] con
    join (select [EmployeeId], max(validto) as maxvalidto
        FROM [EmployeeContract]
        group by [EmployeeId]) mvt
    on con.[EmployeeId] = mvt.[EmployeeId] and mvt.maxvalidto = con.validto) maxv
join 
    (select distinct con.[EmployeeId], mvf.minvalidfrom
    from [EmployeeContract] con
    join (select [EmployeeId], min(validfrom) as minvalidfrom
        FROM [EmployeeContract]
        group by [EmployeeId]) mvf
    on con.[EmployeeId] = mvf.[EmployeeId] and mvf.minvalidfrom = con.validfrom) minv
on minv.[EmployeeId] = maxv.[EmployeeId]
order by 1

但我并不满意，我认为它不容易阅读，并且可能优化很差。我怎样才能做得更好？

Answer 1

我想你想要group by:

select employeeid, min(validfrom), max(validto)
from employeecontract
group by employeeid