## 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.

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## Solution

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#### Approach I: Using flow control statement `CASE` [Accepted]

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Algorithm

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For students with odd id, the new id is (id+1) after switch unless it is the last seat. And for students with even id, the new id is (id-1). In order to know how many seats in total, we can use a subquery:

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`SELECT\n    COUNT(*) AS counts\nFROM\n    seat\n`
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Then, we can use the `CASE` statement and `MOD()` function to alter the seat id of each student.

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MySQL

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`SELECT\n    (CASE\n        WHEN MOD(id, 2) != 0 AND counts != id THEN id + 1\n        WHEN MOD(id, 2) != 0 AND counts = id THEN id\n        ELSE id - 1\n    END) AS id,\n    student\nFROM\n    seat,\n    (SELECT\n        COUNT(*) AS counts\n    FROM\n        seat) AS seat_counts\nORDER BY id ASC;\n`
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#### Approach II: Using bit manipulation and `COALESCE()` [Accepted]

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Algorithm

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Bit manipulation expression `(id+1)^1-1` can calculate the new id after switch.

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`SELECT id, (id+1)^1-1, student FROM seat;\n`
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`| id | (id+1)^1-1 | student |\n|----|------------|---------|\n| 1  | 2          | Abbot   |\n| 2  | 1          | Doris   |\n| 3  | 4          | Emerson |\n| 4  | 3          | Green   |\n| 5  | 6          | Jeames  |\n`
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Then, we can make a temp table and join seat with this table like below.

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`SELECT\n    *\nFROM\n    seat s1\n        LEFT JOIN\n    seat s2 ON (s1.id+1)^1-1 = s2.id\nORDER BY s1.id;\n`
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`| id | student | id | student |\n|----|---------|----|---------|\n| 1  | Abbot   | 2  | Doris   |\n| 2  | Doris   | 1  | Abbot   |\n| 3  | Emerson | 4  | Green   |\n| 4  | Green   | 3  | Emerson |\n| 5  | Jeames  |    |         |\n`
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Note:The first two columns are from s1 and the last two are from s2.

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At last, we can output s1.id and s2.student. However, the s2.student is NULL for seat id \'5\' but s1.student is right. Thus, we we can use function `COALESCE()` to generate the correct output for the last record.

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MySQL

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`SELECT\n    s1.id, COALESCE(s2.student, s1.student) AS student\nFROM\n    seat s1\n        LEFT JOIN\n    seat s2 ON ((s1.id + 1) ^ 1) - 1 = s2.id\nORDER BY s1.id;\n`
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Note: This solution comes from @FANGXIAOFANG.

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