一条sql统计出个阶段的数量

需求案例：统计出各年龄段人数

表结构：

sql：

SELECT
	count( a1 ) AS A1,
	count( a2 ) AS A2,
	count( a3 ) AS A3 
FROM
	(
SELECT
	( CASE WHEN age <= 18 THEN age END ) AS a1,
	( CASE WHEN age > 18 AND age <= 22 THEN age END ) AS a2,
	( CASE WHEN age > 22 THEN age END ) AS a3 
FROM
	sys_user 
	) t

结果：