create table bricks (
colour varchar2(10),
shape varchar2(10),
weight integer
);
insert into bricks values ( 'red', 'cube', 1 );
insert into bricks values ( 'red', 'pyramid', 2 );
insert into bricks values ( 'red', 'cuboid', 1 );
insert into bricks values ( 'blue', 'cube', 1 );
insert into bricks values ( 'blue', 'pyramid', 2 );
insert into bricks values ( 'green', 'cube', 3 );
commit;
count() 返回查询处理的行数，显示砖表中有多少行
select count ( colour ) from bricks;
select sum ( weight ), min ( weight ), max ( weight ),
avg ( weight ), stddev ( weight ),
median ( weight ), variance ( weight ),
stats_mode ( weight )
from bricks;
查找颜色列中不同值的数量。共有三种颜色（红色、绿色和蓝色）
select count ( distinct colour ) number_of_different_colours
from bricks;
关键字 all 明确告诉函数处理每一行。您还可以使用 unique 作为 distinct 的同义词
select count ( all colour ) total_number_of_rows,
count ( distinct colour ) number_of_different_colours,
count ( unique colour ) number_of_unique_colours
from bricks;
您还可以在大多数统计函数中使用 distinct，例如 sum 和 avg。
select sum ( weight ) total_weight, sum ( distinct weight ) sum_of_unique_weights,
avg ( weight ) overall_mean, avg ( distinct weight ) mean_of_unique_weights
from bricks;
分组聚合
将结果分成不同的组。您可以通过 group by 执行此操作
select colour, count (*)
from bricks
group by colour;
这可能会令人困惑，因此最好在选择中包含所有分组列。
将引发异常，因为形状在选择中，但不在组中
select colour, shape, count (*)
from bricks
group by colour;
按多列分组。以下返回每个形状和重量的行数(脑瓜疼)
select shape, weight, count (*)
from bricks
group by shape, weight;
过滤聚合
只能在 where 子句中过滤未聚合的值。如果在此处包含聚合函数，则会出现错误
select colour, count (*)
from bricks
where weight > 1
group by colour;
这样就不对了
要过滤聚合函数，请使用 having 子句。这样写是错误的
select colour, count (*)
from bricks
where count (*) > 1
group by colour;
正确的写法
having 子句可以在 group by 之前或之后进行
select colour, count (*)
from bricks
group by colour
having count (*) > 1;
select colour, count (*)
from bricks
having count (*) > 1
group by colour;
Rollup 为其中的列生成小计
汇总（颜色，形状）
select colour, shape, count (*)
from bricks
group by rollup ( colour, shape );
select colour, shape, count (*)
from bricks
group by colour, rollup ( shape );
