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solve problem Exchange Seats

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`@@ -331,4 +331,5 @@ All solutions will be accepted!`
`331`	`331`	`\|183\|[Customers Who Never Order](https://leetcode-cn.com/problems/customers-who-never-order/description/)\|[mysql](./database/CustomersWhoNeverOrder)\|Easy\|`
`332`	`332`	`\|197\|[Rising Temperature](https://leetcode-cn.com/problems/rising-temperature/description/)\|[mysql](./database/RisingTemperature)\|Easy\|`
`333`	`333`	`\|181\|[Employees Earning More Than Their Managers](https://leetcode-cn.com/problems/employees-earning-more-than-their-managers/description/)\|[mysql](./database/EmployeesEarningMoreThanTheirManagers)\|Easy\|`
`334`		`-\|176\|[Second Highest Salary](https://leetcode-cn.com/problems/second-highest-salary/description/)\|[mysql](./database/SecondHighestSalary)\|Easy\|`
	`334`	`+\|176\|[Second Highest Salary](https://leetcode-cn.com/problems/second-highest-salary/description/)\|[mysql](./database/SecondHighestSalary)\|Easy\|`
	`335`	`+\|626\|[Exchange Seats](https://leetcode-cn.com/problems/exchange-seats/description/)\|[mysql](./database/ExchangeSeats)\|Medium\|`

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	`1`	`+# Exchange Seats`
	`2`	`+We can solve problem this problem by CASE function, or use union to solve it like below:`
	`3`	+```sql
	`4`	`+# Write your MySQL query statement below`
	`5`	`+select id, student from (`
	`6`	`+ select id - 1 as id, student from seat where mod(id, 2) = 0 union`
	`7`	`+ select id + 1 as id, student from seat where mod(id, 2) = 1 and id != (select count(*) from seat) union`
	`8`	`+ select id, student from seat where mod(id, 2) = 1 and id = (select count(*) from seat)`
	`9`	`+) as s order by id`
	`10`	+```

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	`1`	`+# Write your MySQL query statement below`
	`2`	`+select (`
	`3`	`+ case`
	`4`	`+ when mod(id, 2) = 1 and id = counts then id`
	`5`	`+ when mod(id, 2) = 1 and id < counts then id + 1`
	`6`	`+ else id - 1`
	`7`	`+ end) as id, student from seat, (select count(*) as counts from seat) as seat_counts order by id`

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