Mybatis的动态Sql

 mybatis核心 对sql语句进行灵活操作，通过表达式进行判断，对sql进行灵活拼接、组装。###

 1、动态sql

  映射文件：

DELETE FROM user



and id=#{id}


and username=#{username}

 2、sql片段

  在Mapper文件中可以定义sql片段，定义后sql语句可以引用该片段，达到重复使用的效果。
  映射文件：

and id=#{id}


and username=#{username}


  引用片段：

DELETE FROM user

 3、foreach

  向sql传递数组或List，mybatis使用foreach解析.

  POJO文件：

public class UserPlus extends User{

private List list;

public List getList() {
return list;
}

public void setList(List list) {
this.list = list;
}
}
  映射文件：

SELECT * FROM user id=#{user_id}
  接口文件：

//根据多个id查询多个用户
public List selectUser(UserPlus userPlus) throws Exception;

  测试文件：

public void selectUser(){
try {
sqlSession = sessionFactory.openSession();
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
UserPlus userPlus = new UserPlus();
List list = new ArrayList();
list.add(1);
list.add(3);
list.add(5);
list.add(7);
userPlus.setList(list);
List list1 = userMapper.selectUser(userPlus);
System.out.println("查询成功");
for (User user:list1) {
System.out.println(user);
}
}catch (Exception e){
System.out.println("查询失败");
}finally {
sqlSession.close();
}
}