在使用逆向工程生成代码时，可选择生成实体类和xxxExample类，xxxExample的作用是什么呢
Example实例解析
example用于添加条件，相当where后面的部分
xxxExample example = new xxxExample(); Criteria criteria = new Example().createCriteria();
应用举例
逆向工程生成的文件XxxExample.java中包含一个static的内部类Criteria，Criteria中的方法是定义SQL 语句where后的查询条件。
1.查询
① selectByPrimaryKey()
User user = XxxMapper.selectByPrimaryKey(100); //相当于select * from user where id = 100
② selectByExample()
UserExample example = new UserExample();
Criteria criteria = example.createCriteria();
criteria.andUsernameEqualTo("wyw"); //where条件
criteria.andUsernameIsNull(); //where条件
example.setOrderByClause("username asc,email desc");
List<?>list = XxxMapper.selectByExample(example);
//相当于：select * from user where username = 'wyw' and username is null order by username asc,email desc
2.更新数据
updateByExample()
UserExample example = new UserExample();
Criteria criteria = example.createCriteria();
criteria.andUsernameEqualTo("admin"); //where条件
User user = new User();
user.setPassword("wyw");
XxxMapper.updateByPrimaryKeySelective(user,example);
//相当于：update user set password='wyw' where username='admin'
3.删除数据
deleteByExample()
UserExample example = new UserExample();
Criteria criteria = example.createCriteria();
criteria.andUsernameEqualTo("admin");
XxxMapper.deleteByExample(example);
//相当于：delete from user where username='admin'
4.查询数据数量
countByExample()
UserExample example = new UserExample();
Criteria criteria = example.createCriteria();
criteria.andUsernameEqualTo("wyw");
int count = XxxMapper.countByExample(example);
//相当于：select count(*) from user where username='wyw'
