SQL解析工具JSQLParser

> selectItems = plainSelect.getSelectItems();

获取select语句的where结构
```java
Expression where = plainSelect.getWhere();

获取查询的from后的结构（表，子查询等）

FromItem fromItem = plainSelect.getFromItem();

存在连接查询时，获取连接查询（left/right/inner）join后的结构

List joins = plainSelect.getJoins();

2.9 SetOperationList

用于表示多个select语句通过union，union all连接在一起的联合查询SQL对象

select...from...
union all
select...from...
union all
select...from...

将语句拆分，获取构成它的若干select “`java SetOperationList operationList = (SetOperationList) selectBody; List