[N-0] add 626

Author: fishercoder1534

README.md

@@ -656,6 +656,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |  #  |      Title     |   Solutions   | Time          | Space         | Difficulty  | Tag          | Notes
 |-----|----------------|---------------|---------------|---------------|-------------|--------------|-----
 |627|[Swap Salary](https://leetcode.com/problems/swap-salary/)|[Solution](../master/database/_627.sql) | | | Easy |
+|626|[Exchange Seats](https://leetcode.com/problems/exchange-seats/)|[Solution](../master/database/_626.sql) | | | Medium |
 |620|[Not Boring Movies](https://leetcode.com/problems/not-boring-movies/)|[Solution](../master/database/_620.sql) | | | Easy |
 |619|[Biggest Single Number](https://leetcode.com/problems/biggest-single-number/)|[Solution](../master/database/_619.sql) | | | Easy |
 |618|[Students Report By Geography](https://leetcode.com/problems/students-report-by-geography/)|[Solution](../master/database/_618.sql) | | | Hard | Session Variables

database/_626.sql

@@ -0,0 +1,44 @@
+--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.
+
+
+SELECT
+    (CASE
+        WHEN MOD(id, 2) != 0 AND counts != id THEN id + 1
+        WHEN MOD(id, 2) != 0 AND counts = id THEN id
+        ELSE id - 1
+    END) AS id,
+    student
+FROM
+    seat,
+    (SELECT
+        COUNT(*) AS counts
+    FROM
+        seat) AS seat_counts
+ORDER BY id ASC;