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测试数据

数据库 SQL测试数据 – 笑虾原创诗词表

查询返回单个结果

返回单个 Map

设置返回值类型 resultType="java.util.Map"

PoemMapper.xml

    <select id="selectMap" resultType="java.util.Map">
        SELECT id, title, author FROM poem LIMIT 1   
    </select>

PoemMapper.java

以Map为最外层容器时就要加 @MapKey("id") 指定提取 id 作为 key。

    @MapKey("id")
    Map<Long, Object> selectMap();

PoemMapperTest.java

    @Test
    public void selectMap() { 
   
        Map<Long, Object> map = poemMapper.selectMap();
        System.out.println(JSON.toJSONString(map));
    }  

输出结果

注意：Map是无序的，所以这里的字段并没有按SQL中的顺序来显示。

{ 
   "1":{ 
   "author":"笑虾","id":1,"title":"痴情癫"}}

返回单个 LinkedHashMap

用来实现按SQL中的顺序来显示字段。

PoemMapper.xml

设置返回值类型 resultType="java.util.LinkedHashMap" 。

    <select id="selectLinkedHashMap" resultType="java.util.LinkedHashMap">
        SELECT id, title, author FROM poem LIMIT 1
    </select>

PoemMapper.java

LinkedHashMap<String, Object> selectLinkedHashMap();    

PoemMapperTest.java

LinkedHashMap<String, Object> linkedHashMap = poemMapper.selectLinkedHashMap();

输出结果

注意：LinkedHashMap中字段按SQL中的顺序显示。

{ 
   "id":1,"title":"痴情癫","author":"笑虾"}

查询返回多个结果

1. 用List保留住SQL中ORDER By的排序。
2. 用LinkedHashMap保留住SQL中SELECT 字段的排序。

返回 List＜Map＞

PoemMapper.xml

    <select id="selectMapList" resultType="java.util.Map">
        SELECT id, title, author FROM poem
    </select>

PoemMapper.java

List<Map<String, Object>> selectMapList();

PoemMapperTest.java

List<Map<String, Object>> maps = poemMapper.selectMapList();

输出结果

[
	{ 
   "author":"笑虾","id":1,"title":"痴情癫"},
	{ 
   "author":"笑虾","id":2,"title":"爱云说"},
	{ 
   "author":"笑虾","id":3,"title":"恨灯小"}, 
	略。。。
]

返回 Map＜Map＞

PoemMapper.xml

    <select id="selectMapMap" resultType="java.util.Map">
        SELECT id, title, author FROM poem 
    </select>

PoemMapper.java

@MapKey("id")
Map<String, Map<String, Object>> selectMapMap();

PoemMapperTest.java

Map<String, Map<String, Object>> stringMapMap = poemMapper.selectMapMap();

输出结果

{ 
   
	"1":{ 
   "author":"笑虾","id":1,"title":"痴情癫"},
	"2":{ 
   "author":"笑虾","id":2,"title":"爱云说"},
	"3":{ 
   "author":"笑虾","id":3,"title":"恨灯小"},
	略。。。
}

返回 List＜LinkedHashMap＞

PoemMapper.xml

    <select id="selectListLinkedHashMap" resultType="java.util.LinkedHashMap">
        SELECT id, title, author FROM poem
    </select>

PoemMapper.java

List<LinkedHashMap<String, Object>> selectListLinkedHashMap();    

PoemMapperTest.java

List<LinkedHashMap<String, Object>> linkedHashMap = poemMapper.selectListLinkedHashMap();

输出结果

[
	{ 
   "id":1,"title":"痴情癫","author":"笑虾"},
	{ 
   "id":2,"title":"爱云说","author":"笑虾"},
	{ 
   "id":3,"title":"恨灯小","author":"笑虾"},
	略。。。
]

返回 Map＜LinkedHashMap＞

PoemMapper.xml

    <select id="selectMapLinkedHashMap" resultType="java.util.LinkedHashMap">
        SELECT id, title, author FROM poem
    </select>

PoemMapper.java

@MapKey("id")
Map<String, LinkedHashMap<String, Object>> selectMapLinkedHashMap();

PoemMapperTest.java

Map<String, LinkedHashMap<String, Object>> mapLinkedHashMaps = poemMapper.selectMapLinkedHashMap();

输出结果

{ 
   
	"1":{ 
   "id":1,"title":"痴情癫","author":"笑虾"},
	"2":{ 
   "id":2,"title":"爱云说","author":"笑虾"},
	"3":{ 
   "id":3,"title":"恨灯小","author":"笑虾"},
	略。。。
}

统计结果返回 List<Pair<Integer, Long>> 再转 Map<Integer, Long>>

PoemMapper.xml

    <select id="countByAuthor" resultType="javafx.util.Pair">
        SELECT author,	count( id ) AS `数量` FROM	poem GROUP BY author
    </select>

PoemMapper.java

List<Pair<Integer, Long>> countByAuthor();

PoemMapperTest.java

List<Pair<Integer, Long>> list = poemMapper.countByAuthor();
Map<Integer, Long> map = list.stream()
        .collect(Collectors.toMap(Pair::getKey, Pair::getValue));

输出结果

查询结果返回的是这样的一个List<Pair>。用List<Map>实现也可以。

[{ 
   "key":"笑虾","value":16},{ 
   "key":"金小侠","value":3}]

还需要用Collectors.toMap转为Map才得到我们想要的最终效果。

{ 
   "笑虾":16,"金小侠":3}

参考资料

笑虾：Mybatis 查询结果返回 Optional＜T＞

javafx.util.Pair<K,V>
《Java8实战》 – 读书笔记 – Stream流操作2：用流收集数据：6.1 收集器简介 – toMap