说明：

　　SQLite 的UNOIN 子句/运算符用于合并两个或多个SELECT语句的结果，不返回重复的行。
　　若用UNION，每个SELECT被选择的列数必须是相同的，相同数目的列表达式，相同的数据类型，并确保它们有相同的顺序，但可以的不同的长度。

　　假设有如下表student：

ID          NAME        AGE         ADDRESS
----------  ----------  ----------  ----------
1           xiaoming    18          shenzhen
2           xiaohua     18          beijing
3           mingming    19          shanghai
4           xiaogang    20          hangzhou
5           honghong    17          shenzhen
6           liangliang  21          wuhan
7           tingting    23          shenzhen

　　和表2 region：

  ID          ADDRESS
----------  ----------
1           shenzhen
2           beijing
3           shanghai

UNION语法：

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

UNION

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

实例：

sqlite>select region.ID, NAME from student inner join region on student.ADDRESS=region.ADDRESS 
union
select region.ID, NAME from student left outer join region on student.ADDRESS=region.ADDRESS;
ID          NAME
----------  ----------
            liangliang
            xiaogang
1           honghong
1           tingting
1           xiaoming
2           xiaohua
3           mingming

UNION ALL语法：

　　UNION ALL 运算符用于结合两个 SELECT 语句的结果，包括重复行。适用于 UNION 的规则同样适用于 UNION ALL 运算符。
　　语法如下：

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

UNION ALL

SELECT column1 [, column2 ]
FROM table1 [, table2 ]
[WHERE condition]

实例：

sqlite>select region.ID, NAME from student inner join region on student.ADDRESS=region.ADDRESS 
union all
select region.ID, NAME from student left outer join region on student.ADDRESS=region.ADDRESS;
ID          NAME
----------  ----------
1           honghong
1           tingting
1           xiaoming
2           xiaohua
3           mingming
1           xiaoming
2           xiaohua
3           mingming
            xiaogang
1           honghong
            liangliang
1           tingting