626. Exchange Seats
Mary is a teacher in a middle school and she has a table seat storing students' names and their corresponding seat ids.
The column id is continuous increment.
Mary wants to change seats for the adjacent students.
Can you write a SQL query to output the result for Mary?
+---------+---------+
| id | student |
+---------+---------+
| 1 | Abbot |
| 2 | Doris |
| 3 | Emerson |
| 4 | Green |
| 5 | Jeames |
+---------+---------+For the sample input, the output is:
+---------+---------+
| id | student |
+---------+---------+
| 1 | Doris |
| 2 | Abbot |
| 3 | Green |
| 4 | Emerson |
| 5 | Jeames |
+---------+---------+Note:
If the number of students is odd, there is no need to change the last one’s seat.
Hint: Be aware that select count(*) as cnt from seat, NOT select count(*) from seat as cnt!
Solution:
select if(cnt%2=1 and id=cnt, id, if(id%2=1,id+1,id-1)) as id, student from seat, (select count(*) as cnt from seat) t order by id;
