SQL-第一个表中的所有值与第二个表中的第一个值连接在一起，否则为null

Question

is it possible to do a join like this in SQL:

We want to join the two tables on SessionID, but the resulting table should return full table "Pageviews" combined with only a single value of Sessions for each unique SessionID (for others the value should be NULL, please see the 'Desired Result' table for explanation).

Thank you.

Answer 1

Based on you requirement( and truly based on the data provided) the below code would do the work -

declare @pageviews table(sessionid char(10), pageviews tinyint)
declare @sessions table(sessionid char(10), sessions tinyint)
insert into @pageviews values
('FA-1',34),
('FA-1',36),
('FA-2',23),
('FA-3',11),
('FA-3',32),
('FA-3',25)
insert into @sessions values
('FA-1',23),
('FA-2',14),
('FA-3',9)

;with cte as
(select sessionid,pageviews,ROW_NUMBER() OVER(PARTITION BY sessionid ORDER BY sessionid desc) rn
from @pageviews)
select p.sessionid,p.pageviews,case when rn = 1 then s.sessions else null end as sessions
from cte p inner join @sessions s
on p.sessionid = s.sessionid

Answer 2

You can do like this.

SELECT T2.SessionID,T2.pageviews,T1.Sessions
FROM sessions AS T1
RIGHT JOIN 
(
  SELECT ROW_NUMBER() OVER(PARTITION BY sessionid ORDER BY sessionid DESC) AS ROWNUM,
         sessionid,
         pageviews
  FROM pageviews
) AS T2
ON T1.sessionid = T2.sessionid AND T2.ROWNUM = 1

SQLFiddle