Du måste lägga din första räkning i en underfråga:
SELECT COUNT(CASE WHEN C = 1 THEN 1 END) AS nooffamiliesHavingcount1,
COUNT(CASE WHEN C = 2 THEN 1 END) AS nooffamiliesHavingcount2
FROM ( SELECT COUNT(*) AS C
FROM TableName
GROUP BY House_No
) t
WHERE c IN (1, 2);
REDIGERA
Om du behöver göra intervall i din räkning kan du använda detta:
SELECT COUNT(CASE WHEN C <= 1 THEN 1 END) AS nooffamiliesHavingcount1,
COUNT(CASE WHEN C BETWEEN 2 AND 4 THEN 1 END) AS nooffamiliesHavingcount2,
COUNT(CASE WHEN C > 4 THEN 1 END) AS nooffamiliesHavingcount3
FROM ( SELECT COUNT(*) AS C
FROM TableName
GROUP BY House_No
) t