从数据表中选择每个ID的顶部条目-SQL

Question

Assuming I have the following data table:

ID       VALUE 
---      ------- 
123      a
123      b
456      c
456      d
789      e

How can I get the output to only select the top entry per ID, assuming the data is already ordered. Ie, I need the data to show:

ID       VALUE
---      -------
123      a
456      c
789      e

Answer 1

SQL tables represent unordered sets. There is no ordering to the table, unless you have a column that specifies the ordering.

This is fundamental in SQL. Let me assume you have an ordering column. Then you can do:

select t.*
from t
where t.ordcol = (select max(t2.ordcol) from t t2 where t2.id = t.id);

Answer 2

Also you can add the ordering dynamically,

eg

select * from 

(select id, value, dense_rank() OVER (ORDER BY value asc) as myrank from #mytable ) aa

where myrank=1