select tb.userId from ( select a._user_id as userId, CAST((FLOOR(IFNULL(b.star, 0) / 100) + FLOOR(IFNULL(b.flower, 0) / 50) + FLOOR(IFNULL(b.thumb, 0) / 5) + FLOOR(IFNULL(b.bean, 0))) - FLOOR(IFNULL(b.consumed_points, 0)) as signed) as sortOrder from divx_social.osf_team_members a left join divx_social.dcp_stat_user_achieve b on a._user_id = b.user_id and b.stat_time_type = 4 where a._project_id = 9408 and a._enabled = 1 and a._role_id = 6 and b.inst_id = 17 ) as tb order by sortOrder DESC；解释这段SQL

这是一段SQL查询语句，其作用是从多个表中查询符合条件的记录，并根据计算得到的排序值进行降序排序，最终返回用户ID。

具体解释如下：

1. 查询的表

divx_social.osf_team_members
divx_social.dcp_stat_user_achieve

2. 查询条件

a._project_id = 9408  -- 对应osf_team_members表的项目ID为9408
a._enabled = 1      -- 对应osf_team_members表的启用状态为1
a._role_id = 6      -- 对应osf_team_members表的角色ID为6
b.inst_id = 17      -- 对应dcp_stat_user_achieve表的实例ID为17
b.stat_time_type = 4   -- 对应dcp_stat_user_achieve表的时间类型为4

3. 查询的字段

a._user_id as userId   -- osf_team_members表中的_user_id字段，取别名为userId

4. 排序方式

CAST((FLOOR(IFNULL(b.star, 0) / 100) + FLOOR(IFNULL(b.flower, 0) / 50) + FLOOR(IFNULL(b.thumb, 0) / 5) + FLOOR(IFNULL(b.bean, 0))) - FLOOR(IFNULL(b.consumed_points, 0)) as signed) as sortOrder  -- 计算得到的排序值，取别名为sortOrder，并按照该字段进行降序排序

5. 最终结果

SELECT tb.userId  -- 返回userId字段
FROM (
     -- 子查询，查询符合条件的记录，并计算排序值
     SELECT a._user_id as userId, CAST((FLOOR(IFNULL(b.star, 0) / 100) + FLOOR(IFNULL(b.flower, 0) / 50) + FLOOR(IFNULL(b.thumb, 0) / 5) + FLOOR(IFNULL(b.bean, 0))) - FLOOR(IFNULL(b.consumed_points, 0)) as signed) as sortOrder
     FROM divx_social.osf_team_members a
     LEFT JOIN divx_social.dcp_stat_user_achieve b ON a._user_id = b.user_id AND b.stat_time_type = 4
     WHERE a._project_id = 9408 AND a._enabled = 1 AND a._role_id = 6 AND b.inst_id = 17
) AS tb
ORDER BY sortOrder DESC;  -- 按照排序值进行降序排序