SQL 分组相关列

Question

Suppose I have a table with 2 columns like this:

Person1, Person2
David    Jessica
Jessica  David
David    Oz
Oz       David
Guy      Richard
Richard  Guy
Jessica  Oz
Oz       Jessica

and another table with 2 columns:

Person   Last Posted
David    12/8/2016
Jessica  5/10/2016
Oz       23/11/2016
Guy      8/3/2016
Richard  27/6/2016

assuming a players name is unique and will appear only once. how to choose the person with the latest post? A related group for this example is (David ,Jessica, Oz),(Guy, Richard)

The output should be:

Person
Oz
Richard

Answer 1

This one should work. In the first sub-query, called winner, list the person with the most recent post for each pairing. Then join this to a second sub-query, called loser, which lists the person with the least recent post per pairing. The output should return people who are in the winners sub-query but not the losers one. Being in the losers sub-query would demonstrate that someone in your group had a more recent post than you:

SELECT DISTINCT winner.person
FROM
(SELECT
 CASE WHEN t2_1.last_post > t2_2.last_post THEN person1 ELSE person2 END AS person
 FROM t1
 INNER JOIN t2 t2_1 ON t1.person1 = t2_1.person
 INNER JOIN t2 t2_2 ON t1.person2 = t2_2.person) winner
LEFT JOIN
(SELECT
 CASE WHEN t2_1.last_post < t2_2.last_post THEN person1 ELSE person2 END AS person
 FROM t1
 INNER JOIN t2 t2_1 ON t1.person1 = t2_1.person
 INNER JOIN t2 t2_2 ON t1.person2 = t2_2.person) loser
ON winner.person = loser.person
WHERE loser.person IS NULL

Tested here: http://sqlfiddle.com/#!9/6ef390/13

Answer 2

Assuming your first table is table1 and the second is table2, how about this:

select 
Person = case when isnull(t1.[last posted],'1900-01-01')>isnull(t2.[last posted],'1900-01-01') then t.person1 else t.person2 end

from 
table1 t
left join table2 t1 on t.person1=t1.person
left join table2 t2 on t.person2=t2.person

(If you know you won't ever have a person who hasn't posted, you could omit the use of isnull and use inner joins )

Answer 3

This question is not well written. I wrote a solution to the step of selecting distinct users with latest date, but the result comes out different then what you wrote. not sure what kind of logic you aim for.

Join the 2nd table (posts table twice) and then use the greatest function:

select distinct case when posts1.last_posted = greatest(posts1.last_posted,posts2.last_posted) then posts1.person else posts2.person end as latest_person from games left join posts as posts1 on games.person1 = posts1.person left join posts as posts2 on games.person2 = posts2.person