Mysql编程实战三之排序问题
1.需求
使用SQL语句将表中的数据排序。分别有两种排序规则，一种是全局增序；一种相同名次同序但全局增序。
2.表数据
- 建表语句如下：
CREATE TABLE `t` (
`a` char(1) DEFAULT NULL
) ENGINE=InnoDB- 插入数据
INSERT INTO insidemysql.t (a)
VALUES
('x'),
('x'),
('x'),
('y'),
('y'),
('z');3.实现代码
- 全局增序：按照字母顺序排名，但是相同排名依然增序，比如现在有一组字母，排序结果应该如下：
+------+------+
| chara| rank |
+------+------+
| x | 1 |
| x | 2 |
| x | 3 |
| y | 4 |
| y | 5 |
| z | 6 |
+------+------+select
a
,(
select
count(*)
from t t_2
where t_1.a >= t_2.a
)as rank
from t t_1;
+------+------+
| a | rank |
+------+------+
| x | 3 |
| x | 3 |
| x | 3 |
| y | 5 |
| y | 5 |
| z | 6 |
+------+------+
6 rows in set (0.00 sec)可以看到，排序结果并非我们想要的
- 再次修改代码如下
select
a
,case when @field = a
then @rank
else @rank := @rank + 1 end as rank
,@field := a
from
(
select
a
from t
) as t_1,
(
select
@rank := 0,
@field := null
)as t_2
+------+------+-------------+
| a | rank | @field := a |
+------+------+-------------+
| x | 1 | x |
| x | 1 | x |
| x | 1 | x |
| y | 2 | y |
| y | 2 | y |
| z | 3 | z |
+------+------+-------------+
6 rows in set (0.00 sec)还不是想要的结果
select
a
,@rank := @rank + 1 as rank
from
(
select
a
from t
) as t_1,
(
select
@rank := 0,
@field := null
)as t_2
+------+------+
| a | rank |
+------+------+
| x | 1 |
| x | 2 |
| x | 3 |
| y | 4 |
| y | 5 |
| z | 6 |
+------+------+
6 rows in set (0.00 sec)这么来看，这个SQL像是解决了问题，但是真的是这样么？插入两条数据：
insert into t select 'a';
insert into t select 'z';再次执行这个语句，结果发现出了问题：
+------+------+
| a | rank |
+------+------+
| x | 1 |
| x | 2 |
| x | 3 |
| y | 4 |
| y | 5 |
| z | 6 |
| a | 7 |
| z | 8 |
+------+------+这里没有把a，z按照字母顺序存储，即没有排序，使用order by的SQL如下：
select
a
,@rank := @rank + 1 as rank
from
(
select
a
from t
order by a
) as t_1,
(
select
@rank := 0,
@field := null
)as t_2;
+------+------+
| a | rank |
+------+------+
| a | 1 |
| x | 2 |
| x | 3 |
| x | 4 |
| y | 5 |
| y | 6 |
| z | 7 |
| z | 8 |
+------+------+
8 rows in set (0.00 sec)从结果看，没有问题。但是接着问题又来了，现在的需求是：
- 2.相同名次同序但全局增序：按照字母顺序排名，但是相同的字母有着相同的排名，比如说，现在有一组字符，他们的排序结果应该如下：
+------+------+
| chara| rank |
+------+------+
| x | 1 |
| x | 1 |
| x | 1 |
| y | 4 |
| y | 4 |
| z | 6 |
+------+------+select
a
,case when @field != a
then @rank := @rank + 1
else @rank
end as rank
,@field := a
from
(
select
a
from t
order by a
) as t_1,
(
select
@rank := 0,
@field := null
)as t_2;执行结果如下：
+------+------+-------------+
| a | rank | @field := a |
+------+------+-------------+
| a | 0 | a |
| x | 1 | x |
| x | 1 | x |
| x | 1 | x |
| y | 2 | y |
| y | 2 | y |
| z | 3 | z |
| z | 3 | z |
+------+------+-------------+
8 rows in set (0.00 sec)这个SQL实现了相同字母相同排序的功能，但是没有实现全局增量排序，修改SQL如下：
select
a
,case when @field = a
then @rank := @rank
else @rank := @ord end as rank
,@field := a
,@ord := @ord +1
from
(
select
a
from t
order by a
) as t_1,
(
select
@rank := 1,
@ord := 1,
@field := null
)as t_2;+------+------+-------------+-----------------+
| a | rank | @field := a | @ord := @ord +1 |
+------+------+-------------+-----------------+
| a | 1 | a | 2 |
| x | 2 | x | 3 |
| x | 2 | x | 4 |
| x | 2 | x | 5 |
| y | 5 | y | 6 |
| y | 5 | y | 7 |
| z | 7 | z | 8 |
| z | 7 | z | 9 |
+------+------+-------------+-----------------+
8 rows in set (0.00 sec)最终排序成功。
