ピボットから値を集計する方法

counted2 を変更するのが最も速い方法のようです。 CTE、列 TimesTested あなたの論理を考慮してください。

counted2 as (
  SELECT
    client_id,
    CASE WHEN TimesTested >= 12 THEN 12 ELSE TimesTested END TimesTested,
    CAST(COUNT(*) AS varchar(30)) AS count,
    CAST(AVG(testfreq) as varchar(30)) as TestFreq,
    CAST(STDEV(TestFreq) as varchar(30)) Stdv
  FROM counted
  GROUP BY
    client_id,
    CASE WHEN TimesTested >= 12 THEN 12 ELSE TimesTested END
    )