在使用mybatis中指定字段查詢

1：需求：查詢學過“葉平”老師所教的所有課的同學的學號、姓名；

List<Map<String,Object>> selectYepingAllCourse(@Param("name") String name);

用

List<Map<String,Object>>來接收從數據庫查詢的數據

<!--學葉平老師教的所有課的學生的學號，姓名-->
<select id="selectYepingAllCourse" resultMap="Base_ResultMap">
            SELECT
            a.s_id,
            a.sname
        FROM
            a_student a
        WHERE
            a.s_id IN (
                SELECT
            a.s_id
        FROM
            a_sc a
        INNER JOIN a_course b ON a.c_id = b.c_id
        INNER JOIN a_teacher c ON c.t_id = b.t_id
        WHERE
            c.tname = #{name}
        GROUP BY
            a.s_id
        HAVING
            count(a.c_id) = (
                SELECT
                    count(a_course.c_id)
                FROM
                    a_course
                INNER JOIN a_teacher ON a_teacher.t_id = a_course.t_id
                WHERE
                    a_teacher.tname = #{name}
            )
            );
</select>

分析一下這個sql。

1：都有哪些學生學習了葉平老師的課。
    SELECT
    a.s_id,count(a.c_id)
    FROM
    a_sc a
    INNER JOIN a_course b ON a.c_id = b.c_id
    INNER JOIN a_teacher c ON c.t_id = b.t_id
            WHERE
    c.tname = '葉平'
    GROUP BY
    a.s_id
    2：葉平老師教了幾門課。
    SELECT
            count(a_course.c_id)
        FROM
            a_course
        INNER JOIN a_teacher ON a_teacher.t_id = a_course.t_id
        WHERE
            a_teacher.tname = '葉平'
    )
    3：學葉平老師課和葉平老師教的課一樣的學生的信息。
    SELECT
    a.s_id
FROM
    a_sc a
INNER JOIN a_course b ON a.c_id = b.c_id
INNER JOIN a_teacher c ON c.t_id = b.t_id
WHERE
    c.tname = '葉平'
GROUP BY
    a.s_id
HAVING
    count(a.c_id) = (
        SELECT
            count(a_course.c_id)
        FROM
            a_course
        INNER JOIN a_teacher ON a_teacher.t_id = a_course.t_id
        WHERE
            a_teacher.tname = '葉平'
    )

測試：

/**
     * 查詢學過“葉平”老師所教的所有課的同學的學號、姓名；
     */
    @Test
    public void testSelectYepingAllCourse(){
        List<Map<String, Object>> list = ascMapper.selectYepingAllCourse("李一");
        for (Map<String,Object> i:list){
            System.out.println(i);
        }
    }

輸出的結果：

得到List集合中Map集合的值。

for(int i=0;i<list.size();i++){
            Map map=list.get(i);
            //是用set來接收的。不能重復。無順序
            Set set = map.keySet();
            System.out.println("set的值 "+set);
            Object sId = map.get("sId");
            System.out.println("得到sId的值 "+sId);
        }

輸出結果：這個只有一條結果。

下面看下多個結果的。