SQL statement sorting

1.Add a sequence number column before each result extracted by the SELECT statement, starting from 1. The step size is...

Select (select count (*) from blog a where a. ID <= B. ID) as unid, * From Blog B
2. Tips for using groupby order by top

 

Code
Dedeclare @ t table (ID Int , Dat datetime, userid Int )
Insert into @ t select 1 , Getdate (), 1  
Union all select 2 , Getdate () + 1 , 2
Union all select 3 , Getdate () + 3 , 3
Union all select 3 , Getdate () + 10 , 3
Union all select 5 , Getdate () + 9 , 5

Select top3Userid, max (DAT)AsDat from @ T group by userid order by dat DESC
Execution sequence: group by, Max, order by, and top
Explanation: first, use group by to group users, then use Max to retrieve the unique userids with the maximum time in each group, and then sort these userids in descending order by time.