聚合函数

注意：聚合函数会自动忽略空值（null）

六、group by 分组

1、语法格式

select distinct 字段名1,字段名2,… from 表名 where 筛选条件 group by 分组字段 having 筛选条件 order by 排序字段 排序规则 limit n,m;

示例：

-- 住宿和不住宿的各多少人    分组 住宿一组，不住宿一组 分别计数
SELECT board,COUNT(*) FROM gy_stu WHERE board IS NOT NULL GROUP BY board;

3、having 分组后筛选

-- 统计每个班大专本科人数
select class_name,education,count(*) from sl_stu  where education in ("大专", "本科") GROUP BY class_name,education;
select class_name,education,count(*) from sl_stu  GROUP BY class_name,education having education in ("大专", "本科");