现有表结构：
CREATE TABLE `example_dataset` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`tel` bigint(11) DEFAULT NULL,
`gender` varchar(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4;
插入数据
INSERT INTO `example_dataset` (`id`, `tel`, `gender`)
VALUES
(2, 7136609221, 'male'), --'male'是男，'female'是女
(3, 7136609222, 'male'), (6, 7136609222, 'male'), (4, 7136609223, 'female'), (7, 7136609223, 'female'), (5, 7136609228, 'male');
现在sql记录里面有重复的。我想统计表中有多少人，多少男的，多少女的。
SELECT COUNT(DISTINCT tel, gender) as gender_count,
COUNT(DISTINCT tel, gender = 'male') as man_count,
SUM(if(gender = 'female', 1, 0)) as woman_count
FROM example_dataset;
显然结果是不对的。
SELECT COUNT(*) gender_count,
SUM(IF(gender='male',1,0)) male_count,
SUM(IF(gender='female',1,0)) female_count
FROM (
SELECT DISTINCT tel, gender
FROM example_dataset
) t
这样结果是对的。但是我想如果example_dataset表的数据量比较大，子查询会影响效率。
最终答案：
SELECT COUNT(DISTINCT tel) gender_count,
COUNT(DISTINCT CASE WHEN gender = 'male' THEN tel END) male_count,
COUNT(DISTINCT CASE WHEN gender = 'female' THEN tel END) female_count
FROM example_dataset
