MySQL：更新条件和选择语句

Question

I have table with id, degree, roundAbout and action. I need to update action based on next row's degree and action value. To explain, something like this needs to be done. If current degree is less than next row degree and roundAbout is 'N' action should be set to 'TurnRight' and if current degree is greater than next row degree and roundAbout is 'N' action should be set to 'TurnLeft' and if nxt row roundAbout is 'y' then irrespective of degree action should be set to 'FollowRoundAbout'

How do i achieve this ? Here is the example of my table

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

Answer 1

This sounds like a case expression with window functions:

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;

You can save this result into a new table (or view) or update the existing table if you have an action column.

Answer 2

For MySql 8.0+ you can do it with LEAD() window function and a self join:

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; 

See the demo .
Results:

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

Answer 3

Solution: 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

Solution 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