MySQL：更新条件和选择语句答案

【问题标题】：MySQL: Update on condition and select statementMySQL：更新条件和选择语句
【发布时间】：2020-11-17 14:10:53
【问题描述】：

我有带有 id、degree、roundAbout 和 action 的表格。我需要根据下一行的程度和动作值更新动作。为了解释，需要做这样的事情。如果当前度数小于下一行度数且 roundAbout 为“N”，则操作应设置为“TurnRight”，如果当前度数大于下一行度数且 roundAbout 为“N”，则操作应设置为“TurnLeft”，如果 nxt行 roundAbout 为 'y' 那么无论度数动作都应设置为 'FollowRoundAbout'

我如何做到这一点？这是我的表的例子

ID   |   degree  |  roundabout|
----------------------------
 1   |    30     |  N         |
 2   |   130     |  N         |
 3   |   330     |  N         |
 4   |   210     |  N         |
 5   |    30     |  Y         |

My expected table would be

ID   |   degree  |  roundabout|   action|
-----------------------------------------
 1   |    30     |  N         |   RIGHT
 2   |   130     |  N         |   RIGHT
 3   |   330     |  N         |   LEFT
 4   |   210     |  N         |   FollowRound
 5   |    30     |  Y         |   Stop

【问题讨论】：

如果您不使用 MySql 8.0+，您可以使用滚动变量来存储前一行的 degree 和 roundabout 值，同时通过 @987654324 对表进行迭代@

标签： mysql sql aggregate

【解决方案1】：

对于 MySql 8.0+，您可以使用 LEAD() 窗口函数和自联接：

UPDATE tablename t1
INNER JOIN (
  SELECT *, 
         LEAD(degree) OVER (ORDER by ID) next_degree,
         LEAD(roundabout) OVER (ORDER by ID) next_roundabout
  FROM tablename
) t2 ON t2.id = t1.id
SET t1.action = CASE
  WHEN t2.next_roundabout = 'Y' THEN 'FollowRound'
  WHEN t1.degree < t2.next_degree AND t1.roundAbout = 'N' THEN 'RIGHT'
  WHEN t1.degree > t2.next_degree AND t1.roundAbout = 'N' THEN 'LEFT'
  ELSE 'STOP'
END;

请参阅demo。
结果：

> ID | degree | roundabout | action     
> -: | -----: | :--------- | :----------
>  1 |     30 | N          | RIGHT       
>  2 |    130 | N          | RIGHT       
>  3 |    330 | N          | LEFT      
>  4 |    210 | N          | FollowRound
>  5 |     30 | Y          | STOP

【讨论】：

【解决方案2】：

这听起来像是带有窗口函数的case 表达式：

select t.*,
       (case when next_roundabout = 'Y'
             then 'Follow Roundabout'
             when degree < next_degree and roundabout = 'N'
             then 'TurnRight'
             when degree > next_degree and roundabout = 'N'
             then 'TurnLeft'
             when next_degree is null
             then 'Stop'
        end) as action
from (select t.*,
             lead(degree) over (order by id) as next_degree,
             lead(roundabout) over (order by id) as next_roundabout
      from t
     ) t;

如果您有 action 列，您可以将此结果保存到新表（或视图）中或更新现有表。

【讨论】：

【解决方案3】：

解决方案：1

select a.id, a.degree, a.roundabout, case when  a.degree < b.degree and b.roundabout = 'N' then  
'RIGHT'    
when  a.degree > b.degree and b.roundabout = 'N'
THEN 'LEFT'
when  b.roundabout = 'Y' then 'FollowRound'
end
as action
from table1 a crossjoin table2 b
where b.id = a.id + 1

解决方案 2：

select a.id, a.degree, a.roundabout, case when  a.degree < b.degree and b.roundabout = 'N' then  
    'RIGHT'    
    when  a.degree > b.degree and b.roundabout = 'N'
    THEN 'LEFT'
    when  b.roundabout = 'Y' then 'FollowRound'
    end
    as action from table a join table b on two.id = one.id + 1

【讨论】：