在这种情况下，您可以使用子查询:

In this case you can use a subquery:

SELECT
  COUNT(*),
  CASE
    WHEN age >=10 AND age <=20 THEN '10-20'
    WHEN age >=21 AND age <=30 THEN '21-30'
    WHEN age >=31 AND age <=40 THEN '31-40'
    WHEN age >=41 AND age <=50 THEN '41-50'
    WHEN age >=51 AND age <=60 THEN '51-60'
    WHEN age >=61 THEN '61+'
  END AS ageband
FROM
  (
     DATE_FORMAT(NOW(), '%Y') - DATE_FORMAT(date_of_birth, '%Y') - (DATE_FORMAT(NOW(), '00-%m-%d') < DATE_FORMAT(date_of_birth, '00-%m-%d')) AS age,
.. ..
  ) as tbl
GROUP BY ageband;

因此，首先，它执行子查询并建立年龄表，然后再汇总年龄值.

So first it executes the subquery and builds a table of ages, than it aggregates the age value.