MyBatis五、MyBatis的各种查询功能

MyBatis的各种查询功能

一、查询一个实体类对象

1. 其结果可被接收的方式

   可通过实体类对象接收

   可通过List集合接收：即第二种情况

   可通过Map结构：即第四种情况

2. Mapper接口

   public interface SelectMapper {
       User getUserById(@Param("id") Integer id);
   }
   

3. Mapper映射文件

   <mapper namespace="com.atguigu.mybatis.mapper.SelectMapper">
       <select id="getUserById" resultType="user">
           select * from t_user where id = #{id}
       select>
   mapper>
   

4. 测试

   @Test
   public void getUserByIdTest() {
       SqlSession sqlSession = SqlSessionUtils.getSqlSession();
       SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
       User userById = mapper.getUserById(1);
       // 输出结果：User{id=1, username='admin', pwd='123456', age=23, sex=男, email='[email protected]'}
       System.out.println(user);
   }
   

二、查询一个List集合

1. 其结果可被接收的方式

   可通过List集合接收

   可通过Map结构

2. Mapper接口

   public interface SelectMapper {
       List<User> getAllUser();
   }
   

3. Mapper映射文件

   <mapper namespace="com.atguigu.mybatis.mapper.SelectMapper">
       <select id="getAllUser" resultType="user">
           select * from t_user 
       select>
   mapper>
   

4. 测试

   @Test
       public void getAllUserTest() {
           SqlSession sqlSession = SqlSessionUtils.getSqlSession();
           SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
           List<User> allUser = mapper.getAllUser();
           allUser.forEach(user -> System.out.println(user));
       }
   

三、查询单个数据

1. mybatis自带的typeAlias

   Type	Integer	String	Map	List	Double	Iterator
   Alias	int/integer	string	map	list	double	iterator

2. Mapper接口

   public interface SelectMapper {
       int getCount();
   }
   

3. Mapper映射文件

   注：一定要有resultType

   <select id="getCount" resultType="Integer">
       select count(*) from t_user
   select>
   

4. 测试

   @Test
   public void getCountTest() {
       SqlSession sqlSession = SqlSessionUtils.getSqlSession();
       SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
       int count = mapper.getCount();
       System.out.println("count = " + count);
   }
   

四、查询一条数据为Map集合

1. mapper接口

   public interface SelectMapper {
       Map<String,Object> getUserByIdToMap(@Param("id") Integer id);
   
   }
   

2. mapper映射文件

   <select id="getUserByIdToMap" resultType="map">
       select * from t_user where id = #{id}
   select>
   

3. 测试

   @Test
   public void getUserByIdToMapTest() {
       SqlSession sqlSession = SqlSessionUtils.getSqlSession();
       SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
       Map<String, Object> userByIdToMap = mapper.getUserByIdToMap(1);
       // 输出结果：{sex=男, id=1, pwd=123456, age=23, [email protected], username=admin}
       System.out.println(userByIdToMap);
   }
   

五、查询多条数据为Map集合

1. @MapKey()

   用来设置Map集合的键值对

   会将查询出来的某条数据的某一个字段作为Map集合的键，

   会将查询出来的某条数据作为Map集合的值

2. Mapper接口

   public interface SelectMapper {
       @MapKey("id")
       Map<String,Object> getAllToMap();
   }
   

3. Mapper映射文件

   <select id="getAllToMap" resultType="map">
       select * from t_user
   select>
   

4. 测试

   @Test
   public void getAllToMapTest() {
       SqlSession sqlSession = SqlSessionUtils.getSqlSession();
       SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
       Map<String, Object> allToMap = mapper.getAllToMap();
       /*	输出结果：
       	{1={sex=男, id=1, pwd=123456, age=23, [email protected], username=admin},
           2={sex=男, id=2, pwd=123456, age=23, [email protected], username=admin}, 
           3={sex=男, id=3, pwd=123456, age=23, [email protected], username=admin}, 
           4={sex=男, id=4, pwd=123456, age=23, [email protected], username=李四},
           10={sex=女, id=10, pwd=123456, age=23, [email protected], username=张三}}
   
       */
       System.out.println(allToMap);
   }