表结构：
CREATE TABLE test.testsort ( id int(11) NOT NULL AUTO_INCREMENT, uid int(11) DEFAULT 0 COMMENT '用户id', score decimal(10, 2) DEFAULT 0.00 COMMENT '分数', PRIMARY KEY (id) ) ENGINE = INNODB AUTO_INCREMENT = 1
CHARACTER SET utf8 COLLATE utf8_general_ci COMMENT = '测试排序' ROW_FORMAT = DYNAMIC;
思路：可以先排序，再对结果进行编号；也可以先查询结果，再排序编号。
说明：
@rownum := @rownum + 1 中 := 是赋值的作用，这句话的意思是先执行@rownum + 1，然后把值赋给@rownum；
(SELECT @rownum := 0) r 这句话的意思是设置rownum字段的初始值为0，即编号从1开始。
实现排名：
方法一：
SELECT t.*, @rownum := @rownum + 1 AS rownum FROM (SELECT @rownum := 0) r, (SELECT * FROM testsort ORDER BY score DESC) AS t;
方法二：
SELECT t.*, @rownum := @rownum + 1 AS rownum FROM (SELECT @rownum := 0) r, testsort AS t ORDER BY t.score DESC;
结果：
查看指定用户排名：
方法一：
SELECT b.* FROM ( SELECT t.*, @rownum := @rownum + 1 AS rownum FROM (SELECT @rownum := 0) r, (SELECT * FROM testsort ORDER BY score DESC) AS t ) AS b WHERE b.uid = 222;
方法二:
SELECT b.* from ( SELECT t.*, @rownum := @rownum + 1 AS rownum FROM (SELECT @rownum := 0) r, testsort AS t ORDER BY t.score DESC ) as b where b.uid = 222;
结果：
实现从指定用户uid为111,222,333,444,555 中获取uid为222的排名
SELECT b.* FROM ( SELECT t.*, @rownum := @rownum + 1 AS rownum FROM (SELECT @rownum := 0) r, (SELECT * FROM testsort WHERE uid IN (111,222,333,444,555) ORDER BY score DESC) AS t ) AS b WHERE b.uid = 222;
结果：
