SQL Joins：选择员工提交评论的状态以及每年未提交评论的员工列表

Question

对于每一年，对于每位员工，我想列出该员工已提交的评论的状态，或者“未启动”以防员工未提交该年度的评论。

这个问题很难用语言表达，所以我会尝试通过举例来解释它：

create table #employees 
(
empid int,
name varchar(100)
)

Create table #review
(
empid int,
ryear int,
status varchar(20)
)

insert into #review values(1,2016,'S2')
insert into #review values(2,2016,'S2')
insert into #review values(2,2017,'S1')
insert into #review values(3,2017,'S2')



insert into #employees values(1,'jack')
insert into #employees values(2,'mack')
insert into #employees values(3,'rack')
insert into #employees values(4,'tack')

查询错误

select a.empid
      ,a.name
      ,b.ryear
      ,case isnull(b.status,'')
           when ''
               then 'Not Initiated'
           else status
       end as status
from #employees as a
    left join #review as b
        on a.empid = b.empid
           and b.ryear in(select distinct
                                 ryear
                          from #review
                         );--something like that

预期结果：

+-------+------+-------+----------------+
| empid | name | ryear |     status     |
+-------+------+-------+----------------+
|     1 | jack |  2016 | S2             |
|     1 | jack |  2017 | not initiated  |
|     2 | mack |  2016 | S2             |
|     2 | mack |  2017 | S1             |
|     3 | rack |  2016 | not initieated |
|     3 | rack |  2017 | S2             |
|     4 | tack |  2016 | Not Initiated  |
|     4 | tack |  2017 | Not Initiated  |
+-------+------+-------+----------------+

Answer 1

这是一个使用公用表表达式的例子； 它还支持员工在同一年内进行多次审核。

WITH X AS
(SELECT distinct ryear FROM #review)
SELECT a.empid, a.name, X.ryear, isnull(b.status,'Not initiated')
  FROM X as x
  LEFT
  JOIN #employees as a
    ON 1=1
  LEFT
  JOIN #review as b
    ON a.empid = b.empid
   AND b.ryear = x.ryear
 ORDER 
    BY a.empid,x.ryear

Answer 2

据推测，这支持同一员工在同一年中存在多个评论：

SELECT Employees.empid, Employees.[name], ReviewYears.[Value]
     , [Status] = ISNULL(LatestReviews.[status], 'Not Initiated')
FROM #employees AS Employees
CROSS JOIN (SELECT DISTINCT ryear AS [Value] FROM #review) AS ReviewYears -- We need some source of years, hopefully there are no missing years here.
LEFT JOIN (
    SELECT *
    FROM (
        SELECT empid, ryear, [status]
             , RN = ROW_NUMBER() OVER(PARTITION BY empid, ryear ORDER BY STATUS DESC) -- Per employee and year, we'll take only one status, hopefully we can order by statuses.
        FROM #review
    ) AS T
WHERE RN = 1 -- Refer to comments at creation of RN.
) AS LatestReviews ON LatestReviews.empid = Employees.empid AND LatestReviews.ryear = ReviewYears.[Value] -- Refer to comments at creation of RN.
ORDER BY Employees.empid ASC, ReviewYears.[Value] ASC

Answer 3

您可以在子查询中使用交叉连接

select a.empid
      ,a.name
      ,c.ryear
      ,isnull(b.status,'Not Initiated') as status
from #employees as a
    cross join(select distinct
                      ryear
               from #review
              ) as c
    left join #review as b
            on b.ryear = c.ryear
            and a.empid = b.empid
order by a.empid, ryear