group by a：將所有具有相同a字段值的記錄放到一個分組里；
group by a,b ：將所有具有相同a字段值和b字段值的記錄放到一個分組里
count( case when sex='1' then '男') as count1,
count( case when sex='2' then '女') as count2,
使用語法：
SELECT
CASE WHEN salary <= 500 THEN '1'
WHEN salary > 500 AND salary <= 600 THEN '2'
WHEN salary > 600 AND salary <= 800 THEN '3'
WHEN salary > 800 AND salary <= 1000 THEN '4'
ELSE NULL END salary_class, -- 別名命名
COUNT(*) FROM Table_A
GROUP BY
CASE WHEN salary <= 500 THEN '1'
WHEN salary > 500 AND salary <= 600 THEN '2'
WHEN salary > 600 AND salary <= 800 THEN '3'
WHEN salary > 800 AND salary <= 1000 THEN '4'
ELSE NULL END;