改错误原因为sql语句拼写错误

xml的update语句为

<update id="updateStudent" parameterType="java.util.Map">
    update tab_stu
    <if test="stu_name != null">
        stu_name = #{stu_name},
    </if>
    <if test="sex != null">
        sex = #{sex},
    </if>
    <if test="classInfo != null">
        c_id = #{classInfo},
    </if>
    where stu_id = #{stu_id}
</update>

可以看出每句话后面都会有一个，逗号 生成的sql语句则会在where前多一个，即，update tab_stu stu_name = ?, sex = ?, c_id = ?, where stu_id = ? 
此时要使用<trim></trim>标签进行去字符串句末的都好功能即代码改为

<update id="updateStudent" parameterType="java.util.Map">
    update tab_stu
<trim prefix="set" suffixOverrides=",">  <!--此处未来去掉生成在where前的逗号-->
        <if test="stu_name != null">
            stu_name = #{stu_name},
        </if>
        <if test="sex != null">
            sex = #{sex},
        </if>
        <if test="classInfo != null">
            c_id = #{classInfo},
        </if>
    </trim>
    where stu_id = #{stu_id}
</update>
结果生成的sql语句为 update tab_stu set stu_name = ?, sex = ?, c_id = ? where stu_id = ? 

其实<trim></trim>还有可以对字符串头的多余字符进行操作，此处不做演示

mybatis学习中update报错Exception in thread "main" org.apache.ibatis.exceptions.PersistenceException: （三）