MySQL ——group by子句使用with rollup
group by 子句使用with rollup关键字之后，具有分组加和的功能。即：在所有的分组记录之后，自动新增一条记录，从全局计算所有记录的数据。
0 问题描述
求出每年的学生平均成绩，及历史至今的平均成绩，结果保留两位小数。
1 数据准备
create table rollup_test
(
name varchar(8) COMMENT'',
year int COMMENT'',
score int COMMENT''
)CHARACTER SET utf8 COLLATE utf8_general_ci;
insert into rollup_test
values ('a',2016, 85),
('b',2016, 45),
('c',2016, 90),
('a',2015, 75),
('b',2015, 90);2 数据分析
完成代码如下：
select `year`,
round(avg(score) ,2) as avg_score
from rollup_test group by `year`
union all
select '历史至今' as `year`,
round(avg(score) ,2) as avg_score
from rollup_test
方式二：利用group by with rollup 替换union all进行简化
select `year`,
round(avg(score) ,2) as avg_score
from rollup_test
group by `year`
with rollup
由输出结果可知，with rollup 会自动新增一条记录：
year avg_score
null 77.00
结果与下列代码等价
select '历史至今' as `year`,
round(avg(score) ,2) as avg_score
from rollup_test再借助函数ifnull 或 coalesce() 对空字段赋值
上述代码继续优化为：
select coalesce (`year`,'历史至今') as `year`,
round(avg(score) ,2) as avg_score
from rollup_test
group by `year`
with rollup输出结果为：
