因为在很多业务逻辑复杂的项目中，往往不是简单的sql语句就能查询出来自己想要的数据，所有mybatis引入了动态sql语句,

UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.hao.mapper.UserMapper">
   <select id="findByCondition" parameterType="user" resultType="user">
       select * from user
       <where>
           <if test="id!=0">
               and id=#{id}
           </if>
           <if test="username!=null">
               and username=#{username}
           </if>
           <if test="password!=null">
               and password=#{password}
           </if>
       </where>
   </select>
</mapper>

Dao层接口

Mybatis映射文件动态SQL语句-01_mybatis

然后测试

@Test
    public void test1() throws IOException {
        InputStream stream = Resources.getResourceAsStream("SqlMapConfig.xml");
        SqlSession sqlSession = new SqlSessionFactoryBuilder().build(stream).openSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
//      模拟条件
        User user = new User();
        user.setId(1);
        user.setUsername("张三");
        user.setPassword("123");
        List<User> userList = mapper.findByCondition(user);
        System.out.println(userList);
    }

结果：

Mybatis映射文件动态SQL语句-01_xml_02