[Mybatis] Various query functions of MyBatis (check entity class objects, check collections, check single data, and the query result is a map collection)

1. Query an entity class object

 /*
    * 根据用户id查询
    * */
    User GetUserByID(@Param("id") int id);

<!--    User GetUserByID();-->
    <select id="GetUserByID" resultType="com.li.pojo.User">
        select * from USER where id=#{id}
    </select>

@Test
    public void GetUserById(){
        SqlSession sqlSession = GetSqlSession.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        User user = mapper.GetUserByID(1);
        System.out.println(user);
    }

2. Query a list collection

/*
    * 查询所有用户信息
    * */
    List<User> GetAllUser();

<!--    List<User> GetAllUser();-->
    <select id="GetAllUser" resultType="com.li.pojo.User">
        select * from USER
    </select>

 @Test
    public void GetAllUser(){
        SqlSession sqlSession = GetSqlSession.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        List<User> list = mapper.getAllUser();
        list.forEach(System.out::println);
    }

3. Query a single data

/*
    * 查询单个数据,例如总行数
    * */
    Integer GetCount();

<!--        Integer GetCount();-->
    <select id="GetCount" resultType="java.lang.Integer">
        select count(*) from user
    </select>

@Test
    public void GetCount(){
        SqlSession sqlSession = GetSqlSession.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        Integer integer = mapper.GetCount();
        System.out.println(integer);
    }

4. Query a piece of data as a map collection

The properties of the map collection are not fixed. For example, if some properties have no value, they will not be queried

/**
     * 根据用户id查询用户信息为map集合
     */
    Map<String, Object> getUserToMap(@Param("id") int id);

<!--    Map<String, Object> getUserToMap(@Param("id") int id);-->
    <select id="getUserToMap" resultType="map">
        select * from user where id=#{id}
    </select>

@Test
    public void getUserToMap(){
        SqlSession sqlSession = GetSqlSession.getSqlSession();
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        Map<String, Object> map = mapper.getUserToMap(1);
        System.out.println(map);
    }

5. Query multiple pieces of data as a map collection

method 1

/**
* 查询所有用户信息为map集合
* @return
* 将表中的数据以map集合的方式查询，一条数据对应一个map；若有多条数据，就会产生多个map集合，此
时可以将这些map放在一个list集合中获取
*/
List<Map<String, Object>> getAllUserToMap();

<!--Map<String, Object> getAllUserToMap();-->
<select id="getAllUserToMap" resultType="map">
select * from t_user
</select>

Method 2

/**
* 查询所有用户信息为map集合
* @return
* 将表中的数据以map集合的方式查询，一条数据对应一个map；若有多条数据，就会产生多个map集合，并
且最终要以一个map的方式返回数据，此时需要通过@MapKey注解设置map集合的键，值是每条数据所对应的
map集合
*/
@MapKey("id")
Map<String, Object> getAllUserToMap();

<!--Map<String, Object> getAllUserToMap();-->
<!--
{
1={password=123456, sex=男, id=1, age=23, username=admin},
2={password=123456, sex=男, id=2, age=23, username=张三},
3={password=123456, sex=男, id=3, age=23, username=张三}
}
-->
<select id="getAllUserToMap" resultType="map">
select * from t_user
</select>