【leetcode Database】178. Rank Scores

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题目：

Write a SQL query to rank scores. If there is a tie between two scores, both should have the same ranking. Note that after a tie, the next ranking number should be the next consecutive integer value. In other words, there should be no "holes" between ranks.

+----+-------+
| Id | Score |
+----+-------+
| 1  | 3.50  |
| 2  | 3.65  |
| 3  | 4.00  |
| 4  | 3.85  |
| 5  | 4.00  |
| 6  | 3.65  |
+----+-------+

For example, given the above Scores table, your query should generate the following report (order by highest score):

+-------+------+
| Score | Rank |
+-------+------+
| 4.00  | 1    |
| 4.00  | 1    |
| 3.85  | 2    |
| 3.65  | 3    |
| 3.65  | 3    |
| 3.50  | 4    |
+-------+------+

解析：先对Scores表排序，然后声明一个rank字段，当score相等时，rank不变，score不等时，rank+1。代码如下：

 SELECT Score,Rank FROM 
 (SELECT Score,
 @Rank := IF(@prevScore = Score,@Rank,@Rank+1) AS Rank,
 @prevScore := Score FROM Scores
 ORDER BY Score DESC) s, 
 (SELECT @Rank := 0, @prevScore := NULL) r;