Mysql实现排名函数的三种方法
MySQL没有提供排名函数，但是我们可以通过一些技巧来实现开窗函数的效果。
1. 环境搭建、目标结果集
CREATE TABLE `tem` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`str` char(1) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8;
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (1, 'A');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (2, 'B');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (3, 'A');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (4, 'C');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (5, 'A');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (6, 'C');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (7, 'B');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (8, 'A');| num |
id |
str |
|---|---|---|
| 1 | 1 | A |
| 2 | 3 | A |
| 3 | 5 | A |
| 4 | 8 | A |
| 1 | 2 | B |
| 2 | 7 | B |
| 1 | 4 | C |
| 2 | 6 | C |
2.实现方法
2.1 连表查询
SELECT
count(*) num,
t1.*
FROM
tem t1
INNER JOIN tem t2 ON t1.str = t2.str AND t1.id >= t2.id
GROUP BY
t1.id
ORDER BY
t1.str, t1.id;让我们看看这个查询的执行计划
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+
| 1 | SIMPLE | t1 | ALL | PRIMARY | NULL | NULL | NULL | 8 | Using temporary; Using filesort |
| 1 | SIMPLE | t2 | ALL | PRIMARY | NULL | NULL | NULL | 8 | Using where; Using join buffer (flat, BNL join) |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+
2 rows in set (0.06 sec)非常低效：临时表、文件排序、循环嵌套都用上了
2.2 子查询
SELECT
(SELECT COUNT(*) FROM tem t2 WHERE t1.str = t2.str AND t1.id >= t2.id) num,
t1.*
FROM
tem t1
ORDER BY
t1.str, t1.id;再看看执行计划
+----+--------------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+-------+------+---------------+------+---------+------+------+----------------+
| 1 | PRIMARY | t1 | ALL | NULL | NULL | NULL | NULL | 8 | Using filesort |
| 2 | DEPENDENT SUBQUERY | t2 | ALL | PRIMARY | NULL | NULL | NULL | 8 | Using where |
+----+--------------------+-------+------+---------------+------+---------+------+------+----------------+
2 rows in set (0.06 sec)很好，两次全扫描，而且只用到了文件排序，如果加上索引，文件排序也可以避免。
2.3MySQL变量
SELECT
@num := IF(@str = str, @num + 1, 1) num,
id,
@str := str str
FROM
tem, (SELECT @str := '', @num := 0) t1
ORDER BY
str, id;执行计划
+----+-------------+------------+--------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+------+---------+------+------+----------------+
| 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | |
| 1 | PRIMARY | tem | ALL | NULL | NULL | NULL | NULL | 8 | Using filesort |
| 2 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
+----+-------------+------------+--------+---------------+------+---------+------+------+----------------+
3 rows in set (0.06 sec) 效率最高，一次全扫描搞定。文件排序是因为没有索引。