[N-0] add 578

Author: fishercoder1534

README.md

@@ -685,6 +685,7 @@ Your ideas/fixes/algorithms are more than welcome!
 |585|[Investments in 2016](https://leetcode.com/problems/investments-in-2016/)|[Solution](../master/database/_585.java) | || Medium|
 |584|[Find Customer Referee](https://leetcode.com/problems/find-customer-referee/)|[Solution](../master/database/_584.java) | || Easy|
 |580|[Count Student Number in Departments](https://leetcode.com/problems/count-student-number-in-departments/)|[Solution](../master/database/_580.sql) | || Medium | Left Join
+|578|[Get Highest Answer Rate Question](https://leetcode.com/problems/get-highest-answer-rate-question/)|[Solution](../master/database/_578.sql) | || Medium |
 |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 |

database/_578.sql

@@ -0,0 +1,31 @@
+--578. Get Highest Answer Rate Question
+--
+--Get the highest answer rate question from a table survey_log with these columns: uid, action, question_id, answer_id, q_num, timestamp.
+--
+--uid means user id; action has these kind of values: "show", "answer", "skip"; answer_id is not null when action column is "answer", while is null for "show" and "skip"; q_num is the numeral order of the question in current session.
+--
+--Write a sql query to identify the question which has the highest answer rate.
+--
+--Example:
+--Input:
+--+------+-----------+--------------+------------+-----------+------------+
+--| uid  | action    | question_id  | answer_id  | q_num     | timestamp  |
+--+------+-----------+--------------+------------+-----------+------------+
+--| 5    | show      | 285          | null       | 1         | 123        |
+--| 5    | answer    | 285          | 124124     | 1         | 124        |
+--| 5    | show      | 369          | null       | 2         | 125        |
+--| 5    | skip      | 369          | null       | 2         | 126        |
+--+------+-----------+--------------+------------+-----------+------------+
+--Output:
+--+-------------+
+--| survey_log  |
+--+-------------+
+--|    285      |
+--+-------------+
+--Explanation:
+--question 285 has answer rate 1/1, while question 369 has 0/1 answer rate, so output 285.
+--Note: The highest answer rate meaning is: answer number's ratio in show number in the same question.
+
+SELECT question_id AS 'survey_log' FROM survey_log GROUP BY question_id ORDER BY
+COUNT(answer_id) / COUNT(case when survey_log.action =
+ 'show' then survey_log.action else null end) DESC LIMIT 0,1