# SELECT UNION Syntax
#### 常用语法
```
SELECT ...UNION [ALL | DISTINCT] 
SELECT ...[UNION [ALL | DISTINCT] SELECT ...]
```
#### 示例
```
select userid from user union select orderid from ordertbl order by userid;
select userid from user union (select orderid from ordertbl group by orderid) order by userid;
```
#### 语法限制
对于UNION中的每个SELECT，不支持使用多个同名的列。