如何用UNION嵌套Select查询?

你可以用 join 在派生表上。另外,你应该使用 union all union ,因此查询不会产生删除重复项的开销:

select u.user_name, es.*
from ((select User_id, login_date as EventTime, 'Login' AS action
       from Users_Login_Log
       where login_date > '2019-12-23'
      ) union all
      (select User_ID, EventTimeStamp AS EventTime, 'Logout' as action
       from EventTrackLog
       where EventName = 'Default.Logout.Clicked' and 
             EventTimeStamp > '2019-12-23'
      )
     ) es join
     users u
     on es.User_Id = u.User_Id
order by es.User_id desc, es.EventTime desc;

SELECT * FROM
(
select User_id ,login_date AS EventTime,'Login' AS ACTION
from Users_Login_Log(nolock)
where login_date > '2019-12-23 00:00:00.000'

UNION

select
 UserID, EventTimeStamp AS EventTime ,
 'Logout' AS ACTION
 from EventTrackLog(nolock) where  EventName = 'Default.Logout.Clicked' and EventTimeStamp > '2019-12-23 00:00:00.000'

) AS ES
join userTable on es.user_id = userTable.user_id
ORDER BY User_id DESC, EventTime DESC

我建议不要嵌套查询 UNION 但只需将这两组查询连接到user表中,并使用别名对其排序,如下所示:

SELECT ull.User_id AS UserID,
    ull.login_date AS EventTime,
    'Login' AS ACTION
FROM Users_Login_Log(nolock) ull
INNER JOIN usersTable ut ON ut.user_id = ull.user_id
WHERE login_date > '2019-12-23 00:00:00.000'
UNION
SELECT et.UserID, 
    et.EventTimeStamp AS EventTime ,
    'Logout' AS ACTION
FROM EventTrackLog(nolock) et
INNER JOIN usersTable ut ON ut.user_id = et.user_id
WHERE et.EventName = 'Default.Logout.Clicked' 
AND et.EventTimeStamp > '2019-12-23 00:00:00.000'
ORDER BY UserID DESC, EventTime DESC