您需要一个年龄表来填充没有匹配行的条目的结果。这可以通过实际表完成，也可以通过子查询动态生成，如下所示：
SELECT a.ageband, IFNULL(t.agecount, 0)
FROM (
-- ORIGINAL QUERY
SELECT
CASE
WHEN age IS NULL THEN 'Unspecified'
WHEN age < 18 THEN '<18'
WHEN age >= 18 AND age <= 24 THEN '18-24'
WHEN age >= 25 AND age <= 30 THEN '25-30'
WHEN age >= 31 AND age <= 40 THEN '31-40'
WHEN age > 40 THEN '>40'
END AS ageband,
COUNT(*) as agecount
FROM (SELECT age FROM Table1) t
GROUP BY ageband
) t
right join (
-- TABLE OF POSSIBLE AGEBANDS
SELECT 'Unspecified' as ageband union
SELECT '<18' union
SELECT '18-24' union
SELECT '25-30' union
SELECT '31-40' union
SELECT '>40'
) a on t.ageband = a.ageband
演示：http ://www.sqlfiddle.com/#!2/7e2a9/10
602392714
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