MySQL 定义连续值范围的开始点和结束点

备注:测试数据库版本为MySQL 8.0

一.需求

mysql> select * from v;
±--------±-----------±-----------+
| proj_id | proj_start | proj_end |
±--------±-----------±-----------+
| 1 | 2020-10-01 | 2020-10-02 |
| 2 | 2020-10-02 | 2020-10-03 |
| 3 | 2020-10-03 | 2020-10-04 |
| 4 | 2020-10-04 | 2020-10-05 |
| 5 | 2020-10-06 | 2020-10-07 |
| 6 | 2020-10-16 | 2020-10-17 |
| 7 | 2020-10-17 | 2020-10-18 |
| 8 | 2020-10-18 | 2020-10-19 |
| 9 | 2020-10-19 | 2020-10-20 |
| 10 | 2020-10-21 | 2020-10-22 |
| 11 | 2020-10-26 | 2020-10-27 |
| 12 | 2020-10-27 | 2020-10-28 |
| 13 | 2020-10-28 | 2020-10-29 |
| 14 | 2020-10-29 | 2020-10-30 |
±--------±-----------±-----------+

希望得到如下结果集
±---------±-----------±-----------+
| proj_grp | proj_start | proj_end |
±---------±-----------±-----------+
| 1 | 2020-10-01 | 2020-10-05 |
| 2 | 2020-10-06 | 2020-10-07 |
| 3 | 2020-10-16 | 2020-10-20 |
| 4 | 2020-10-21 | 2020-10-22 |
| 5 | 2020-10-26 | 2020-10-30 |
±---------±-----------±-----------+

二.解决方案

首先，必须识别它们的范围。proj_start和proj_end的值定义了一行的范围，要把某行看做“连续的”或者属于一个组，
则它的PROJ_START值一定要等于它前一行的PROJ_END值。

如果某个行的PROJ_START值不等于前一行的PROJ_END值，而且它的PROJ_END值也不等于下一行的PROJ_START值，则它就是一个单独的组。

确定了范围之后，就需要将属于同意范围的行划成同一组，并且只返回组开始和结束的点。

create view v2
as
select a.*,
       case 
          when (
             select b.proj_id
                from v b
              where a.proj_start = b.proj_end
                )
               is not null then 0 else 1
        end as flag
   from v a;


select * from v2;

select proj_grp,
       min(proj_start) as proj_start,
       max(proj_end) as proj_end
   from (
select a.proj_id,a.proj_start,a.proj_end,
       (select sum(b.flag)
           from v2 b
        where b.proj_id <= a.proj_id) as proj_grp
      from v2 a
      ) x
group by proj_grp;

测试记录

mysql>
mysql> create view v2
    -> as
    -> select a.*,
    ->        case
    ->           when (
    ->              select b.proj_id
    ->                 from v b
    ->               where a.proj_start = b.proj_end
    ->                 )
    ->                is not null then 0 else 1
    ->         end as flag
    ->    from v a;
Query OK, 0 rows affected (0.01 sec)

mysql>
mysql> select * from v2;
+---------+------------+------------+------+
| proj_id | proj_start | proj_end   | flag |
+---------+------------+------------+------+
|       1 | 2020-10-01 | 2020-10-02 |    1 |
|       2 | 2020-10-02 | 2020-10-03 |    0 |
|       3 | 2020-10-03 | 2020-10-04 |    0 |
|       4 | 2020-10-04 | 2020-10-05 |    0 |
|       5 | 2020-10-06 | 2020-10-07 |    1 |
|       6 | 2020-10-16 | 2020-10-17 |    1 |
|       7 | 2020-10-17 | 2020-10-18 |    0 |
|       8 | 2020-10-18 | 2020-10-19 |    0 |
|       9 | 2020-10-19 | 2020-10-20 |    0 |
|      10 | 2020-10-21 | 2020-10-22 |    1 |
|      11 | 2020-10-26 | 2020-10-27 |    1 |
|      12 | 2020-10-27 | 2020-10-28 |    0 |
|      13 | 2020-10-28 | 2020-10-29 |    0 |
|      14 | 2020-10-29 | 2020-10-30 |    0 |
+---------+------------+------------+------+
14 rows in set (0.00 sec)

mysql>
mysql> select proj_grp,
    ->        min(proj_start) as proj_start,
    ->        max(proj_end) as proj_end
    ->    from (
    -> select a.proj_id,a.proj_start,a.proj_end,
    ->        (select sum(b.flag)
    ->            from v2 b
    ->         where b.proj_id <= a.proj_id) as proj_grp
    ->       from v2 a
    ->       ) x
    -> group by proj_grp;
+----------+------------+------------+
| proj_grp | proj_start | proj_end   |
+----------+------------+------------+
|        1 | 2020-10-01 | 2020-10-05 |
|        2 | 2020-10-06 | 2020-10-07 |
|        3 | 2020-10-16 | 2020-10-20 |
|        4 | 2020-10-21 | 2020-10-22 |
|        5 | 2020-10-26 | 2020-10-30 |
+----------+------------+------------+
5 rows in set (0.00 sec)

mysql>