Add solution 596 [sql]

Author: yanglbme

README.md

@@ -43,6 +43,7 @@ Complete [solutions](https://github.com/doocs/leetcode/tree/master/solution) to
 | 344 | [Reverse String](https://github.com/doocs/leetcode/tree/master/solution/344.Reverse%20String) | `Two Pointers`, `String` |
 | 581 | [Shortest Unsorted Continuous Subarray](https://github.com/doocs/leetcode/tree/master/solution/581.Shortest%20Unsorted%20Continuous%20Subarray) | `Array` |
 | 595 | [Big Countries](https://github.com/doocs/leetcode/tree/master/solution/595.Big%20Countries) | `SQL` |
+| 596 | [Classes More Than 5 Students](https://github.com/doocs/leetcode/tree/master/solution/596.Classes%20More%20Than%205%20Students) | `SQL` |
 | 605 | [Can Place Flowers](https://github.com/doocs/leetcode/tree/master/solution/605.Can%20Place%20Flowers) | `Array` |
 | 695 | [Max Area of Island](https://github.com/doocs/leetcode/tree/master/solution/695.Max%20Area%20of%20Island) | `Array`, `Depth-first Search` |
 | 703 | [Kth Largest Element in a Stream](https://github.com/doocs/leetcode/tree/master/solution/703.Kth%20Largest%20Element%20in%20a%20Stream) | `Heap` |

solution/596.Classes More Than 5 Students/README.md

@@ -0,0 +1,44 @@
+## 超过5名学生的课
+### 题目描述
+
+有一个 `courses` 表 ，有: **student(学生)** 和 **class(课程)**。
+
+请列出所有超过或等于5名学生的课。
+
+例如,表:
+```
++---------+------------+
+| student | class      |
++---------+------------+
+| A       | Math       |
+| B       | English    |
+| C       | Math       |
+| D       | Biology    |
+| E       | Math       |
+| F       | Computer   |
+| G       | Math       |
+| H       | Math       |
+| I       | Math       |
++---------+------------+
+```
+
+应该输出:
+```
++---------+
+| class   |
++---------+
+| Math    |
++---------+
+```
+
+**Note:**
+学生在每个课中不应被重复计算。
+
+### 解法
+注意学生可能被重复计算，需要 `distinct`。
+
+```sql
+# Write your MySQL query statement below
+select class from courses group by class having count(distinct student) >= 5
+
+```