/*
SUM 求和
CASE WHEN 条件就像where 之后一样的写法
THEN 取谁 (取sex = "1" 的 population 的 SUM合  否则取0结束)
END 结束
*/

SELECT country,
SUM( CASE WHEN sex = ‘1‘ THEN population ELSE 0 END) 男,
SUM( CASE WHEN sex = ‘2‘ THEN population ELSE 0 END) 女
FROM cs_country
GROUP BY country;