Oracle开发者中级第6课（并集差集和交集）实验
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概述
本实验参考DevGym中的实验指南。
创建环境
创建2张表：
create table my_brick_collection (
colour varchar2(10),
shape varchar2(10),
weight integer
);
create table your_brick_collection (
height integer,
width integer,
depth integer,
colour varchar2(10),
shape varchar2(10)
);
insert into my_brick_collection values ( 'red', 'cube', 10 );
insert into my_brick_collection values ( 'blue', 'cuboid', 8 );
insert into my_brick_collection values ( 'green', 'pyramid', 20 );
insert into my_brick_collection values ( 'green', 'pyramid', 20 );
insert into my_brick_collection values ( null, 'cuboid', 20 );
insert into your_brick_collection values ( 2, 2, 2, 'red', 'cube' );
insert into your_brick_collection values ( 2, 2, 2, 'blue', 'cube' );
insert into your_brick_collection values ( 2, 2, 8, null, 'cuboid' );
commit;
数据如下，这两个表的列数量不同：
SQL> select * from my_brick_collection;
COLOUR SHAPE WEIGHT
_________ __________ _________
red cube 10
blue cuboid 8
green pyramid 20
green pyramid 20
cuboid 20
SQL> select * from your_brick_collection;
HEIGHT WIDTH DEPTH COLOUR SHAPE
_________ ________ ________ _________ _________
2 2 2 red cube
2 2 2 blue cube
2 2 8 cuboid
Union
union 运算符将两个或多个表合并为一个结果集。 要使用它，表必须在每个位置具有相同数量的具有匹配数据类型的列。
所以以下的SQL报错：
SQL>
select * from my_brick_collection
union
select * from your_brick_collection;
Error at Command Line : 1 Column : 1
Error report -
SQL Error: ORA-01789: query block has incorrect number of result columns
01789. 00000 - "query block has incorrect number of result columns"
*Cause:
*Action:
以下SQL正确：
SQL>
select colour, shape from my_brick_collection
union
select colour, shape from your_brick_collection;
COLOUR SHAPE
_________ __________
blue cube
blue cuboid
green pyramid
red cube
cuboid
Union All
my_brick_collection表中有2个重复的colour, shape，因此以下SQL的结果小于my_brick_collection表的行数：
SQL> select distinct * from my_brick_collection;
COLOUR SHAPE WEIGHT
_________ __________ _________
red cube 10
blue cuboid 8
green pyramid 20
cuboid 20
默认的union会去掉重复行，要保留重复行，需要使用union all：
select colour, shape from my_brick_collection
union all
select colour, shape from your_brick_collection;
COLOUR SHAPE
_________ __________
red cube
blue cuboid
green pyramid
green pyramid
cuboid
red cube
blue cube
cuboid
8 rows selected.
实际上，默认的union等价于以下SQL：
select distinct * from (
select colour, shape from my_brick_collection
union all
select colour, shape from your_brick_collection
);
两个练习的答案如下，说明union和union all的优先级比order by高，也就是order by会对union的最终结果排序：
select colour from my_brick_collection
union
select colour from your_brick_collection
order by colour;
select shape from my_brick_collection
union all
select shape from your_brick_collection order by shape;
如果希望子查询排序，可以使用inline view，但CTE不行，例如：
select shape from my_brick_collection
union all
select * from (select shape from your_brick_collection order by shape) ;
Set Difference
差集操作返回一个表中存在而不在另一个表中的所有行，这可以用not exists实现。
但列多了写起来就比较麻烦，而且由于null处理不当，很容易出错。例如：
SQL>
select colour, shape from your_brick_collection ybc
where not exists (
select null from my_brick_collection mbc
where ybc.colour = mbc.colour
and ybc.shape = mbc.shape
);
COLOUR SHAPE
_________ _________
blue cube
cuboid
在以上结果中，第2行，也就是包含null的行不应该在结果集中，因为null和null是不能比较的。
正确的写法如下：
select colour, shape from your_brick_collection ybc
where not exists (
select null from my_brick_collection mbc
where ( ybc.colour = mbc.colour or
( ybc.colour is null and mbc.colour is null )
)
and ( ybc.shape = mbc.shape or
( ybc.shape is null and mbc.shape is null )
)
);
COLOUR SHAPE
_________ ________
blue cube
Minus （差集）
用集合操作就简单多了，而且不容易出错：
select colour, shape from my_brick_collection
minus
select colour, shape from your_brick_collection
COLOUR SHAPE
_________ __________
blue cuboid
green pyramid
但还有另一个微妙的区别。 与 union 一样，minus对输出进行了distinct操作。如果你希望看到所有不同的行，还是要用not exists：
select colour, shape from my_brick_collection mbc
where not exists (
select null from your_brick_collection ybc
where ( ybc.colour = mbc.colour or ( ybc.colour is null and mbc.colour is null ) )
and ybc.shape = mbc.shape
);
COLOUR SHAPE
_________ __________
blue cuboid
green pyramid
green pyramid
Intersect（交集）
用not exists也可以实现交集，但注意null的处理：
select colour, shape from your_brick_collection ybc
where exists (
select null from my_brick_collection mbc
where ( ybc.colour = mbc.colour or ( ybc.colour is null and mbc.colour is null ) )
and ybc.shape = mbc.shape
);
COLOUR SHAPE
_________ _________
red cube
cuboid
intersect集合操作简单多了：
select colour, shape from your_brick_collection
intersect
select colour, shape from my_brick_collection;
COLOUR SHAPE
_________ _________
red cube
cuboid
Finding the Difference Between Two Tables (Symmetric Difference)
查找两个表之间的差异，也就是对称差异，需要做以下的操作：
- 使用minus查找在表一中而不是表二中的行
- 使用minus查找在表二中而不是表一中的行
- 将这两个操作的输出用union(all)结合起来
例如：
select * from (
select colour, shape from your_brick_collection
minus
select colour, shape from my_brick_collection
) union all (
select colour, shape from my_brick_collection
minus
select colour, shape from your_brick_collection
);
COLOUR SHAPE
_________ __________
blue cube
blue cuboid
green pyramid
如果还想看的更清楚，可以把表名附上：
select * from (
select colour, shape, 'your_brick' from your_brick_collection
minus
select colour, shape, 'your_brick' from my_brick_collection
) union all (
select colour, shape, 'my_brick' from my_brick_collection
minus
select colour, shape, 'my_brick' from your_brick_collection
);
COLOUR SHAPE 'YOUR_BRIC
---------- ---------- ----------
blue cube your_brick
blue cuboid my_brick
green pyramid my_brick
利用集合操作，以下是另一种方法：
- 将两个表中的所有行与 union (all) 合并
- 使用intersect找到两个表中都存在的值
- 从第一个查询minus第二个查询
SQL如下：
select * from (
select colour, shape from your_brick_collection
union all
select colour, shape from my_brick_collection
) minus (
select colour, shape from my_brick_collection
intersect
select colour, shape from your_brick_collection
);
COLOUR SHAPE
_________ __________
blue cube
blue cuboid
green pyramid
Symmetric Difference with Group By
前两种方法都有一些缺点，😦。 首先，您必须两次读取两个表中的所有行。 其次minus和intersect都去除了重复值（应用了distinct）。 所以你只能在一张表中看到这些值。 不是所有的行。
为了演示此问题，我们在your_brick_collection表中插入1条重复行，即红色立方体：
insert into your_brick_collection values ( 4, 4, 4, 'red', 'cube' );
红色立方体在your_brick_collection中有两行，在my_brick_collection中有1行，因此应该出现在对称差异结果集中。但是却没有：
select * from (
select colour, shape from your_brick_collection
minus
select colour, shape from my_brick_collection
) union all (
select colour, shape from my_brick_collection
minus
select colour, shape from your_brick_collection
);
COLOUR SHAPE
_________ __________
blue cube
blue cuboid
green pyramid
正确的写法如下，而且只需要每个表只需读一次：
select colour, shape, sum ( your_bricks ), sum ( my_bricks )
from (
select colour, shape, 1 your_bricks, 0 my_bricks
from your_brick_collection
union all
select colour, shape, 0 your_bricks, 1 my_bricks
from my_brick_collection
)
group by colour, shape
having sum ( your_bricks ) <> sum ( my_bricks );
COLOUR SHAPE SUM(YOUR_BRICKS) SUM(MY_BRICKS)
_________ __________ ___________________ _________________
red cube 2 1
blue cube 1 0
green pyramid 0 2
blue cuboid 0 1
下面这个SQL可以看到更详细的对称差异信息：
select colour, shape,
case
when sum ( your_bricks ) < sum ( my_bricks ) then 'ME'
when sum ( your_bricks ) > sum ( my_bricks ) then 'YOU'
else 'EQUAL'
end who_has_extra,
abs ( sum ( your_bricks ) - sum ( my_bricks ) ) how_many
from (
select colour, shape, 1 your_bricks, 0 my_bricks
from your_brick_collection
union all
select colour, shape, 0 your_bricks, 1 my_bricks
from my_brick_collection
)
group by colour, shape;
COLOUR SHAPE WHO_HAS_EXTRA HOW_MANY
_________ __________ ________________ ___________
red cube YOU 1
blue cube YOU 1
cuboid EQUAL 0
green pyramid ME 2
blue cuboid ME 1
环境清理
drop table my_brick_collection;
drop table your_brick_collection;
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