add 571

Author: fishercoder1534

README.md

@@ -641,6 +641,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |580|[Count Student Number in Departments](https://leetcode.com/problems/count-student-number-in-departments/)|[Solution](../master/database/_580.sql) | || Medium | Left Join
 |577|[Employee Bonus](https://leetcode.com/problems/employee-bonus/)|[Solution](../master/database/_577.sql) | || Easy |
 |574|[Winning Candidate](https://leetcode.com/problems/winning-candidate/)|[Solution](../master/database/_574.sql) | || Medium |
+|571|[Find Median Given Frequency of Numbers](https://leetcode.com/problems/find-median-given-frequency-of-numbers/)|[Solution](../master/database/_571.sql) | || Hard |
 |570|[Managers with at Least 5 Direct Reports](https://leetcode.com/problems/managers-with-at-least-5-direct-reports/)|[Solution](../master/database/_570.sql) | || Medium |
 |569|[Median Employee Salary](https://leetcode.com/problems/median-employee-salary/)|[Solution](../master/database/_569.sql) | || Hard |
 |262|[Trips and Users](https://leetcode.com/problems/trips-and-users/)|[Solution](../master/database/_262.sql)||| Hard| Inner Join

database/_571.sql

@@ -0,0 +1,34 @@
+--571. Find Median Given Frequency of Numbers
+-- The Numbers table keeps the value of number and its frequency.
+--
+--+----------+-------------+
+--|  Number  |  Frequency  |
+--+----------+-------------|
+--|  0       |  7          |
+--|  1       |  1          |
+--|  2       |  3          |
+--|  3       |  1          |
+--+----------+-------------+
+--
+--In this table, the numbers are 0, 0, 0, 0, 0, 0, 0, 1, 2, 2, 2, 3, so the median is (0 + 0) / 2 = 0.
+--
+--+--------+
+--| median |
+--+--------|
+--| 0.0000 |
+--+--------+
+--
+--Write a query to find the median of all numbers and name the result as median.
+
+select FORMAT(avg(n.Number),4)*1.0 as median
+from Numbers n left join
+(
+select Number, @prev := @count as prevNumber, (@count := @count + Frequency) as countNumber
+from Numbers,
+(select @count := 0, @prev := 0, @total := (select sum(Frequency) from Numbers)) temp order by Number
+) n2
+on n.Number = n2.Number
+where
+(prevNumber < floor((@total+1)/2) and countNumber >= floor((@total+1)/2))
+or
+(prevNumber < floor((@total+2)/2) and countNumber >= floor((@total+2)/2))