查询各年龄段人数

CREATE TABLE `name` (
  `id`   int(11)      NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  `age`  int(11)      DEFAULT NULL,
  `sex`  varchar(255) DEFAULT '1',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1;
INSERT INTO `name` (`id`, `name`, `age`, `sex`) VALUES (1, '张三', 11, '0');
INSERT INTO `name` (`id`, `name`, `age`, `sex`) VALUES (2, '李四', 21, '0');
INSERT INTO `name` (`id`, `name`, `age`, `sex`) VALUES (3, '王五', 21, '1');
INSERT INTO `name` (`id`, `name`, `age`, `sex`) VALUES (4, '赵六', 41, '1');

SELECT
  CASE    
    WHEN ( age >= 10 AND age <= 20 ) THEN '10-20' 
    WHEN ( age >= 21 AND age <= 30 ) THEN '21-30'
    ELSE '30-' 
  END 'age_range',
count(*) AS count 
FROM name 
GROUP BY
  CASE
    WHEN ( age >= 10 AND age <= 20 ) THEN '10-20' 
    WHEN ( age >= 21 AND age <= 30 ) THEN '21-30'
    ELSE '30-' 
  END;