MySql中实现多对一??????

创建一个数据库叫mybatis

创建一个student表

create table mybatis.student
(
    sid   int         not null
        primary key,
    sname varchar(30) not null,
    tid   int         not null,
    constraint student_ibfk_1
        foreign key (tid) references mybatis.teacher (tid)
);

该表中 sid为主键 tid为外键

sql中数据

创建一个teacher表

create table mybatis.teacher
(
    tid   int         not null
        primary key,
    tname varchar(30) null,
    constraint teacher_tname_uindex
        unique (tname)
);

该表中tid为主键

sql中数据

 根据学生的信息获取老师信息，每个老师对应多个学生，学生只对应一个老师

查询sql语句

select sid,sname,tname
from student s,
     teacher t
where s.tid = t.tid;

查询结果

Mybatis使用association语句实现多对一??????

方法一：子查询

studentMapper.xml配置

    <select id="getStudentTeacherList01" resultMap="studentTeacher">
        select *
        from student;
    select>
    <resultMap id="studentTeacher" type="student">
        <association property="t" column="tid" javaType="teacher" select="getTeacher"/>
    resultMap>
    <select id="getTeacher" parameterType="int" resultType="teacher">
        select *
        from teacher
        where tid = #{tid};
    select>

方法二：连表查询

studentMapper.xml配置

    <select id="getStudentTeacherList02" resultMap="st">
        select sid, sname, tname
        from student s,
             teacher t
        where s.tid = t.tid;
    select>
    <resultMap id="st" type="student">


        <association property="t" javaType="teacher">
            <result property="tname" column="tname"/>
        association>
    resultMap>