mysql-----groupby对多个字段进⾏分组group by a：将所有具有相同a字段值的记录放到⼀个分组⾥；
group by a,b ：将所有具有相同a字段值和b字段值的记录放到⼀个分组⾥
count( case when sex='1' then '男') as count1,
groupby分组count( case when sex='2' then '⼥') as count2,
使⽤语法：
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, -- 别名命名
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;

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