MySQL实现类似Oracle的row_number效果

这个表，数据如下：

mysql> SELECT * FROM t1;

+----+----------+-----+

| id | category | num |

+----+----------+-----+

|  1 | a        |   1 |

|  2 | a        |   2 |

|  3 | a        |   3 |

|  4 | a        |   4 |

|  5 | b        |   5 |

|  6 | b        |   1 |

|  7 | c        |   0 |

|  8 | c        |   9 |

|  9 | d        |   0 |

+----+----------+-----+

需求要查询出每种category里面，num第二大的那条记录。比如应该返回：

+----+----------+-----+

| id | category | num |

+----+----------+-----+

|  3 | a        |   3 |

|  6 | b        |   1 |

|  7 | c        |   0 |

+----+----------+-----+

由于mysql数据库比较弱，没有oracle里面的类似row_NUMBER orer()这样的高级分析函数。所以要实现这样的效果还是比较麻烦。

并且效率很差劲。不过还是可以实现的。下面来看看：

C:\>mysql -P3306

Welcome to the MySQL monitor.  Commands end with ; or \g.

Your MySQL connection id is 13

Server version: 5.7.11-log MySQL Community Server (GPL)

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affiliates. Other names may be trademarks of their respective

owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> use test

Database changed

mysql> DROP TABLE IF EXISTS t1;

Query OK, 0 rows affected (0.37 sec)

mysql> CREATE TABLE t1 (

    ->   id INT AUTO_INCREMENT NOT NULL PRIMARY KEY,

    ->   category CHAR(1) NOT NULL,

    ->   num INT NOT NULL DEFAULT 0

    -> ) ;

Query OK, 0 rows affected (0.40 sec)

mysql> desc t1;

+----------+---------+------+-----+---------+----------------+

| Field    | Type    | Null | Key | Default | Extra          |

+----------+---------+------+-----+---------+----------------+

| id       | int(11) | NO   | PRI | NULL    | auto_increment |

| category | char(1) | NO   |     | NULL    |                |

| num      | int(11) | NO   |     | 0       |                |

+----------+---------+------+-----+---------+----------------+

3 rows in set (0.00 sec)

mysql> INSERT INTO t1 (category, num)

    -> VALUES

    ->   ('a', 1),

    ->   ('a', 2),

    ->   ('a', 3),

    ->   ('a', 4),

    ->   ('b', 5),

    ->   ('b', 1),

    ->   ('c', 0),

    ->   ('c', 9),

    ->   ('d', 0) ;

Query OK, 9 rows affected (0.16 sec)

Records: 9  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM t1;

+----+----------+-----+

| id | category | num |

+----+----------+-----+

|  1 | a        |   1 |

|  2 | a        |   2 |

|  3 | a        |   3 |

|  4 | a        |   4 |

|  5 | b        |   5 |

|  6 | b        |   1 |

|  7 | c        |   0 |

|  8 | c        |   9 |

|  9 | d        |   0 |

+----+----------+-----+

9 rows in set (0.00 sec)

mysql>

测试数据有了，怎么返回我们要的效果呢？

mysql> SELECT * FROM t1;

+----+----------+-----+

| id | category | num |

+----+----------+-----+

|  1 | a        |   1 |

|  2 | a        |   2 |

|  3 | a        |   3 |

|  4 | a        |   4 |

|  5 | b        |   5 |

|  6 | b        |   1 |

|  7 | c        |   0 |

|  8 | c        |   9 |

|  9 | d        |   0 |

+----+----------+-----+

9 rows in set (0.00 sec)

mysql> SELECT

    ->   t1.`id`,

    ->   t1.`category`,

    ->   t1.`num`,
    ->   (SELECT

    ->     COUNT(*)

    ->   FROM

    ->     t1 inner_t1

    ->   WHERE inner_t1.category = t1.`category`

    ->     AND inner_t1.num >= t1.`num`) AS ct

    -> FROM

    ->   t1;

+----+----------+-----+------+

| id | category | num | ct   |

+----+----------+-----+------+

|  1 | a        |   1 |    4 |

|  2 | a        |   2 |    3 |

|  3 | a        |   3 |    2 |

|  4 | a        |   4 |    1 |

|  5 | b        |   5 |    1 |

|  6 | b        |   1 |    2 |

|  7 | c        |   0 |    2 |

|  8 | c        |   9 |    1 |

|  9 | d        |   0 |    1 |

+----+----------+-----+------+

9 rows in set (0.00 sec)

这个效率不行，对于每条记录都回去描述一次原表。再提取出ct=2的记录即可：

mysql> SELECT

    ->   ttmp_1.id,

    ->   ttmp_1.category,

    ->   ttmp_1.num

    -> FROM

    ->   (SELECT

    ->     t1.`id`,

    ->     t1.`category`,

    ->     t1.`num`,

    ->     (SELECT

    ->       COUNT(*)

    ->     FROM

    ->       t1 inner_t1

    ->     WHERE inner_t1.category = t1.`category`

    ->       AND inner_t1.num >= t1.`num`) AS ct

    ->   FROM

    ->     t1) AS ttmp_1

    -> WHERE ttmp_1.ct = 2

    -> ORDER BY ttmp_1.category ASC

    -> ;

+----+----------+-----+

| id | category | num |

+----+----------+-----+

|  3 | a        |   3 |

|  6 | b        |   1 |

|  7 | c        |   0 |

+----+----------+-----+

3 rows in set (0.00 sec)

mysql>

完成。