In order to cooperate with the test, two tables were specially built, and some test data were added, in which the characters of Soochow were repeatedly recorded.
Table: Person_1 Wei Guo characters
Table: Person_2 People in Shu Kingdom
A. Union forms union
Union can join two or more result sets to form a "union". All records in the subresult set are grouped together to form a new result set.
1. Restrictions
If you use Union to join the result set, there are 4 qualifications.
(1) The sub-result sets must have the same structure.
(2) The number of columns in the word result set must be the same.
(3) The data types corresponding to the sub-result sets must be compatible.
(4) Each sub-result set cannot contain order by and compute clauses.
2. Grammatical form
select_statement union [all] select_statement
all means that the final result set will contain all the rows, and duplicate rows cannot be deleted.
Example:
SELECT Name FROM Person_1 UNION SELECT Name FROM Person_2
The result generated is:
Noticing the duplicate records, Sun Quan and Zhou Yu only showed one. Let's replace UNION with UNION ALL to see what the result is:
SELECT Name FROM Person_1 UNION ALL SELECT Name FROM Person_2
注意到重复记录，孙权与周瑜出现了两次，这就是UNION ALL 与 UNION的不同之处。
B、Except形成差集
Except可以对两个或多个结果集进行连接，形成“差集”。返回左边结果集合中已经有的记录，而右边结果集中没有的记录。
限定条件：
1、子结果集要具有相同的结构。
2、子结果集的列数必须相同。
3、子结果集对应的数据类型必须可以兼容。
4、每个子结果集不能包含order by 和 compute子句。
语法形式：
select_statement except select_statement
自动删除重复行。
示例：
SELECT Name FROM Person_1 EXCEPT SELECT Name FROM Person_2
结果：
留意到表Person_2有的，孙权周瑜已被去除。
C、InterSect形成交集
InterSect可以对两个或多个结果集进行连接，形成“交集”。返回左边结果集和右边结果集中都有的记录。
1、限定条件
要是用Except来连接结果集，有4个限定条件。
(1)、子结果集要具有相同的结构。
(2)、子结果集的列数必须相同。
(3)、子结果集对应的数据类型必须可以兼容。
(4)、每个子结果集不能包含order by或compute子句。
2、语法形式
select_statement intersect select_statement
示例：
SELECT Name FROM Person_1 INTERSECT SELECT Name FROM Person_2
返回的结果如下：
留意到只取两张表都有的记录(周瑜，孙权)，这就是所谓的交集。
D、结果集的排序
SELECT Name FROM Person_1 INTERSECT SELECT Name FROM Person_2 ORDER BY Name DESC --此处的字段名相同了，如果不同，请切记排序列名，只能够是第一个表的列名
这里只有两点要注意
1.ORDER BY是对整个运算后的结果排序，并不是对单个数据集。
2.ORDER BY后面排序的字段名称是第一个数据集的字段名或者别名。
转自：http://www.cnblogs.com/kissdodog/archive/2013/06/24/3152743.html