【leetcode】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.
解题思路：这个题目难度不大，第一眼看起来也许觉得有些繁琐，但是如果抽丝剥茧，将解题过程分解成以下几步，就会豁然开朗。
1.把所有奇数行的名字换成相邻的偶数行的名字。
select a.id,b.student from seat a left join seat b on a.id & 1and a.id + 1 = b.id;
2.把所有偶数行的名字换成相邻的奇数行的名字。
select a.id,b.student from seat a left join seat b on a.id & 1 = 0and a.id- 1 = b.id;
3.合并1和2的结果
select a.id,b.studentfrom seat a left join seat b
on(case when a.id & 1 thena.id + 1 = b.idelse a.id -1 = b.id end);
4.如果总行数为奇数，最后一行的名字不用替换。修正SQL如下：
select a.id,case when b.student is null then a.student else b.student endas student from seat a left join seat b
on(case when a.id & 1 thena.id + 1 = b.idelse a.id -1 = b.id end);
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