select distinct teacher from class etc.

select distinct teacher, student etc.

SELECT teacher, COUNT(student) AS amountStudents
FROM ...
GROUP BY teacher;

SELECT * FROM class WHERE teacher IN (SELECT DISTINCT teacher FROM class)

DECLARE @table TABLE (
    [Teacher] [NVarchar](256) NOT NULL ,
    [Student] [NVarchar](256) NOT NULL
)

INSERT INTO @table VALUES ('Teacher 1', 'Student 1')
INSERT INTO @table VALUES ('Teacher 1', 'Student 2')
INSERT INTO @table VALUES ('Teacher 2', 'Student 3')
INSERT INTO @table VALUES ('Teacher 2', 'Student 4')

SELECT
    T.[Teacher],
    (
    	SELECT TOP 1 T2.[Student]
    	FROM @table AS T2
    	WHERE T2.[Teacher] = T.[Teacher]
    ) AS [Student]
FROM @table AS T
GROUP BY T.[Teacher]

Teacher 1, Student 1
Teacher 2, Student 3

SELECT t1.a
        , (SELECT TOP 1 b FROM Table1 t2 WHERE t1.a = t2.a) AS b
        , (SELECT TOP 1 c FROM Table1 t2 WHERE t1.a = t2.a) AS c
        , (SELECT TOP 1 d FROM Table1 t2 WHERE t1.a = t2.a) AS d
    FROM dbo.Table1 t1
    WHERE (your criteria here)
    GROUP BY t1.a

select class_name, count(teacher)
from class group by class_name having count(teacher)=1

select teacher, count(student)
from class group by teacher having count(student)=1

select distinct teacher from class

SELECT DISTINCT ColA FROM Table WHERE ...

Select Distinct column1 -- where your criteria...

Select teacher_id, count(*)
    from teacher_table inner join classes_table
    on teacher_table.teacher_id = classes_table.teacher_id
    group by teacher_id

Select * from tbl
Where ColA in (Select ColA from tbl Group by ColA Having Count(ColA) = 1)

select cola,colb,colc
from yourtable
where cola in
(
  select cola from yourtable where your criteria group by cola having count(*) = 1
)

declare @temp as table (colA nchar, colB nchar, colC nchar, colD nchar, rownum int)

insert @temp (colA, colB, colC, colD, rownum)
select Test.ColA, Test.ColB, Test.ColC, Test.ColD, ROW_NUMBER() over (order by ColA) as rownum
    from Test

select t1.ColA, ColB, ColC, ColD
from @temp as t1
join (
    select ColA, MIN(rownum) [min]
    from @temp
    group by Cola)
 as t2 on t1.Cola = t2.Cola and t1.rownum = t2.[min]