给出不同用户的分数，并且计算排名：
CREATE TABLE test_rank(id int,score int);
INSERT INTO test_rank VALUES (1,10),(2,20),(7,20),(3,30),(4,40),(5,40),(6,50);
mysql> select * from test_rank;
+------+-------+
| id | score |
+------+-------+
| 1 | 10 |
| 2 | 20 |
| 7 | 20 |
| 3 | 30 |
| 4 | 40 |
| 5 | 40 |
| 6 | 50 |
+------+-------+
7 rows in set (0.00 sec)
mysql> set @prev_value := NULL;
Query OK, 0 rows affected (0.00 sec)mysql> set @rank_count := 0;
Query OK, 0 rows affected (0.00 sec)
假设比较到第n行，设置一个变量prev_value,用于存放第n-1行score的分数；用于比较第n行和第n-1行的score，pre_value可以理解为临时保存第n-1行的变量：
rank_count用于保存当前的排名：
mysql> set @prev_value := NULL;
Query OK, 0 rows affected (0.00 sec)mysql> set @rank_count := 0;
Query OK, 0 rows affected (0.00 sec)mysql> SELECT-> id,-> score,-> CASE-> WHEN @prev_value = score THEN ##相等则prev_value不变（第一次为null，不会相等，所以跳转到下一个when语句）-> @rank_count-> WHEN @prev_value := score THEN ##不等，则第n行的score赋值(：=）给prev_value.且rank_count增加1-> @rank_count := @rank_count + 1-> END AS rank_column #case 开始end结束-> FROM-> test_rank-> ORDER BY-> score DESC;
+------+-------+-------------+
| id | score | rank_column |
+------+-------+-------------+
| 6 | 50 | 1 |
| 4 | 40 | 2 |
| 5 | 40 | 2 |
| 3 | 30 | 3 |
| 2 | 20 | 4 |
| 7 | 20 | 4 |
| 1 | 10 | 5 |
+------+-------+-------------+
7 rows in set (0.01 sec)
