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Get first row for one group [closed]
GV |   Class|  SUM
GV1|   L001 |  5000
GV1|   L002 |  5000
GV1|   L003 |  5000
GV2|   L002 |  7000
GV2|   L003 |  7000
GV2|   L001 |  7000
GV3|   L001 |  8000
GV3|   L002 |  8000
GV3|   L003 |  8000

select teacherid
    from mytable
group by teacherid;

select teacherid, ...other cols...
    from (select teacherid, ...other cols...
                 row_number() over (
                     partition by teacherid
                     order by classid /* or class as per edit */) as row_num
            from mytable) my_derived_table
   where my_derived_table.row_num = 1;

WITH T AS (
    SELECT yourTable.*, ROW_NUMBER() OVER(PARTITION BY TeacherID
      ORDER BY ClassID) AS RN
    FROM yourTable
)
SELECT *
FROM T
WHERE RN = 1

create table #t
(
    GV  varchar(4),
    Class varchar(4),
    [SUM] int
)

insert into #t(GV, Class, [SUM])values('GV1', 'L001', 5000)
insert into #t(GV, Class, [SUM])values('GV1', 'L002', 5000)
insert into #t(GV, Class, [SUM])values('GV1', 'L003', 5000)
insert into #t(GV, Class, [SUM])values('GV2', 'L002', 7000)
insert into #t(GV, Class, [SUM])values('GV2', 'L003', 7000)
insert into #t(GV, Class, [SUM])values('GV2', 'L001', 7000)
insert into #t(GV, Class, [SUM])values('GV3', 'L001', 8000)
insert into #t(GV, Class, [SUM])values('GV3', 'L002', 8000)
insert into #t(GV, Class, [SUM])values('GV3', 'L003', 8000)

Select GV, class, [sum] from
(
    Select Row_Number() Over(Partition by GV Order by GV ) as RowId, * from #t
)K
Where RowId = 1

drop table #t

GV   class sum
---- ----- ----
GV1  L001  5000
GV2  L002  7000
GV3  L001  8000