mybatis学习4

Posted 2020-11-05 escapist

回顾SQL99中的连接查询

 

一对一

<mapper namespace="cardNamespace">
    <resultMap type="cn.itcast.javaee.mybatis.one2one.Card" id="cardMap">
        <id property="id" column="id" />
        <result property="num" column="num" />
    </resultMap>
</mapper>

<mapper namespace="studentNamespace">
    <resultMap type="cn.itcast.javaee.mybatis.one2one.Student" id="studentMap">
        <id property="id" column="id" />
        <result property="name" column="name"/>
        <association property="card" resultMap="cardNamespace.cardMap"/>
    </resultMap>
    <select id="findById" parameterType="int" resultMap="studentMap">
        select s.id,s.name,c.id,c.num
        from students s inner join cards c
        on s.cid = c.id 
        and s.id = #{id}
    </select>
</mapper>

一对多

<mapper namespace="gradeNamespace">
    <resultMap type="cn.itcast.javaee.mybatis.one2many.Grade" id="gradeMap">
        <id property="id" column="gid" />
        <result property="name" column="gname"/>
        <collection property="studentList" resultMap="studentNamespace.studentMap"/>
    </resultMap>
    <select id="findGradeByName" parameterType="string" resultMap="gradeMap">
        select g.gid,g.gname,s.sid,s.sname
        from grades g,students s
        where g.gid = s.sgid
        and s.sname = #{name}
    </select>
</mapper>

<mapper namespace="studentNamespace">
    <resultMap type="cn.itcast.javaee.mybatis.one2many.Student" id="studentMap">
        <id property="id" column="sid" />
        <result property="name" column="sname"/>
        <association property="grade" resultMap="gradeNamespace.gradeMap"/>
    </resultMap>
    <select id="findAllByName" parameterType="string" resultMap="studentMap">
        select s.sid,s.sname,g.gid,g.gname
        from grades g,students s
        where g.gid = s.sgid
        and g.gname = #{name}
    </select>
</mapper>

多对多

<mapper namespace="studentNamespace">
    <resultMap type="cn.itcast.javaee.mybatis.many2many.Student" id="studentMap">
        <id property="id" column="sid" />
        <result property="name" column="sname"/>
    </resultMap>
    <select id="findStudentByName" parameterType="string" resultMap="studentMap">
        select s.sid,s.sname
        from students s,middles m,courses c
        where s.sid = m.sid 
        and m.cid = c.cid
        and c.cname = #{name}
    </select>
</mapper>

<mapper namespace="courseNamespace">
    <resultMap type="cn.itcast.javaee.mybatis.many2many.Course" id="courseMap">
        <id property="id" column="cid" />
        <result property="name" column="cname"/>
    </resultMap>
    <select id="findCourseByName" parameterType="string" resultMap="courseMap">
        select c.cid,c.cname
        from students s,middles m,courses c
        where s.sid = m.sid 
        and m.cid = c.cid
        and s.sname = #{name}
    </select>
</mapper>