lx-admin-backend/ruoyi-system/src/main/resources/mapper/system/SysUserRoleMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.ruoyi.system.mapper.SysUserRoleMapper">

    <select id="selectUserIdsByRoleId" resultType="Long">
        select u.user_id from sys_user u
        inner join sys_user_role sur
            on u.user_id = sur.user_id and sur.role_id = #{roleId}
    </select>

</mapper>