Teradata How Can I Use Count(Distinct)?

Most (all?) databases do not support using COUNT(DISTINCT ...) as an analytic function. So in this case, I would suggest just joining to a subquery which finds the distinct counts:

SELECT t1.G1, t1.C1, t2.cnt
FROM yourTable t1
INNER JOIN
(
    SELECT G1, COUNT(DISTINCT C1) AS cnt
    FROM yourTable
    GROUP BY G1
) t2
    ON t2.G1 = t1.G1
ORDER BY
    t1.G1, t1.C1;

You can get a windowed count distinct, but you always need two functions, one possible way is:

SELECT G1, C1, max(dr) over (partition by G1) as cnt
FROM 
 (
    SELECT G1, C1,
       dense_rank()
       over (partition by G1
             order by C1) AS dr
    FROM yourTable
    
 ) as dt

Depending on your data and actual query this might perform better than Tim's query :-)

Of course, this can be modified for NULLable columns flagging the 1st occurance:

SELECT G1, C1, sum(flag) over (partition by G1) as cnt
FROM 
 (
    SELECT G1, C1,
       case
         when lag(C1)
              over (partition by G1
                    order by C1) = C1 
         then null
         else C1
       end as flag 
    FROM yourTable        
 ) as dt

You can use the below query:

SELECT count(C1) from TableName  Group by G1

If your database doesn't use count(distinct) as a window function, you can use the handy trick of the sum of dense_rank():

select t.*,
       (-1 +
        dense_rank() over (partition by g1 order by c1 asc) +
        dense_rank() over (partition by g1 order by c1 desc)
       ) as expected
from t;

Given that count(distinct) as a window function is easily implemented this way, I am surprised that many databases do not support this functionality directly.

One nuance: This counts NULL as a valid value. You don't have NULL values in your sample data so I don't think this affects you. But, if you wanted an exact equivalent:

select t.*,
       ( (case when count(*) over (partition by g1) = count(c1) over (partition by g1)
               then -1 else -2
          end) +
        dense_rank() over (partition by g1 order by c1 asc) +
        dense_rank() over (partition by g1 order by c1 desc)
       ) as expected
from t;