mysql - 选择时带有大小写的sql

Question

我正在做这个问题，

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    |
+-------+------+

答案是:

Write your MySQL query statement below

SELECT Score,   
  CASE
        WHEN @prev = Score THEN @rank
        WHEN @prev := Score THEN @rank := @rank + 1
        WHEN @rank := @rank + 1 THEN @rank
  END AS Rank
FROM Scores, (SELECT @rank := 0, @prev := NULL) r
ORDER BY Score DESC

我不明白这部分:

WHEN @rank := @rank + 1 THEN @rank

当这条线可以被触发时，为什么我需要呢？

Answer 1

这一行:

WHEN @rank := @rank + 1 THEN @rank

它正在递增@rank并返回递增的变量。它几乎完全等同于:

ELSE @rank := @rank + 1

(唯一的区别是 NULL 值，但可能该值不是 NULL。)

这是什么逻辑？当表达式非零且非 NULL 时，MySQL 将其解释为 true。递增的正计数器将非零且非 NULL，因此 when 子句为 true。然后返回该值。