mysql查询（同一个表中相邻的两条记录进行运算）

通过添加一条行记录的方法，重新生成一个表，然后在通过可以通过表的连接进行运算。

--通过一个初始值为0的变量@rownum，依次递增1来实现行号SELECT@rownum:=@rownum+1 AS rownum,nameFROM temp,(SELECT @rownum:=0) tempWHERE @rownum<4（条件，没有也可以）;

给出一般性的语句：

SELECT@rownum:=@rownum+1 AS rownum,所需字段FROM 表名,(SELECT @rownum:=0) 表名WHERE @rownum<N(获取多少行);

然后给出测试案例：第一步，创建测试表；

mysql> CREATE TABLE temp(id INT(4),name VARCHAR(20));Query OK, 0 rows affected (0.13 sec)

第二步，插入测试数据；

mysql> INSERT INTO temp VAlUES(1,'robin');Query OK, 1 row affected (0.10 sec)mysql> INSERT INTO temp VAlUES(2,'wentasy');Query OK, 1 row affected (0.05 sec)mysql> INSERT INTO temp VAlUES(3,'justdb');Query OK, 1 row affected (0.08 sec)mysql> INSERT INTO temp VAlUES(4,'wen');Query OK, 1 row affected (0.08 sec)mysql> INSERT INTO temp SELECT * FROM temp;Query OK, 4 rows affected (0.03 sec)Records: 4  Duplicates: 0  Warnings: 0mysql> INSERT INTO temp SELECT * FROM temp;Query OK, 8 rows affected (0.02 sec)Records: 8  Duplicates: 0  Warnings: 0mysql> INSERT INTO temp SELECT * FROM temp;Query OK, 16 rows affected (0.02 sec)Records: 16  Duplicates: 0  Warnings: 0mysql> INSERT INTO temp SELECT * FROM temp;Query OK, 32 rows affected (0.03 sec)Records: 32  Duplicates: 0  Warnings: 0mysql> INSERT INTO temp SELECT * FROM temp;Query OK, 64 rows affected (0.04 sec)Records: 64  Duplicates: 0  Warnings: 0mysql> INSERT INTO temp SELECT * FROM temp;Query OK, 128 rows affected (0.03 sec)Records: 128  Duplicates: 0  Warnings: 0mysql> INSERT INTO temp SELECT * FROM temp;Query OK, 256 rows affected (0.04 sec)Records: 256  Duplicates: 0  Warnings: 0mysql> INSERT INTO temp SELECT * FROM temp;Query OK, 512 rows affected (0.05 sec)Records: 512  Duplicates: 0  Warnings: 0mysql> INSERT INTO temp SELECT * FROM temp;Query OK, 1024 rows affected (0.05 sec)Records: 1024  Duplicates: 0  Warnings: 0

第三步，实现类似Oracle中的rownum效果；

mysql> SELECT->     @rownum:=@rownum+1 AS rownum,name-> FROM temp,->     (SELECT @rownum:=0) temp-> WHERE @rownum<4;+--------+---------+| rownum | name    |+--------+---------+|      1 | robin   ||      2 | wentasy ||      3 | justdb  ||      4 | wen     |+--------+---------+4 rows in set (0.01 sec)

---------------------------------------------------------------------------------------------------MySQL 几乎模拟了 Oracle,SQL Server等商业数据库的大部分功能，函数。但很可惜，到目前的版本(5.1.33)为止,仍没有实现ROWNUM这个功能下面介绍几种具体的实现方法.建立实验环境如下mysql> create table tbl (    ->  id      int primary key,    ->  col     int    -> );Query OK, 0 rows affected (0.08 sec)mysql> insert into tbl values    -> (1,26),    -> (2,46),    -> (3,35),    -> (4,68),    -> (5,93),    -> (6,92);Query OK, 6 rows affected (0.05 sec)Records: 6  Duplicates: 0  Warnings: 0mysql>mysql> select * from tbl order by col;+----+------+| id | col  |+----+------+|  1 |   26 ||  3 |   35 ||  2 |   46 ||  4 |   68 ||  6 |   92 ||  5 |   93 |+----+------+6 rows in set (0.00 sec)1. 直接在程序中实现；这应该算是效率最高的一种，也极为方便。直接在你的开发程序中（PHP/ASP/C/...）等中，直接初始化一个变量nRowNum=0，然后在while 记录集时，nRowNum++; 然后输出即可。2. 使用MySQL变量；在某些情况下，无法通过修改程序来实现时，可以考虑这种方法。缺点，@x 变量是 connection 级的，再次查询的时候需要初始化。一般来说PHP等B/S应用没有这个问题。但C/S如果connection一只保持则要考虑 set @x=0mysql> select @x:=ifnull(@x,0)+1 as rownum,id,col    -> from tbl    -> order by col;+--------+----+------+| rownum | id | col  |+--------+----+------+|      1 |  1 |   26 ||      1 |  3 |   35 ||      1 |  2 |   46 ||      1 |  4 |   68 ||      1 |  6 |   92 ||      1 |  5 |   93 |+--------+----+------+6 rows in set (0.00 sec)3. 使用联接查询（笛卡尔积）缺点，显然效率会差一些。利用表的自联接，代码如下，你可以直接试一下 select a.*,b.* from tbl a,tbl b where a.col>=b.col 以理解这个方法原理。mysql> select a.id,a.col,count(*) as rownum    -> from tbl a,tbl b    -> where a.col>=b.col    -> group by a.id,a.col;+----+------+--------+| id | col  | rownum |+----+------+--------+|  1 |   26 |      1 ||  2 |   46 |      3 ||  3 |   35 |      2 ||  4 |   68 |      4 ||  5 |   93 |      6 ||  6 |   92 |      5 |+----+------+--------+6 rows in set (0.00 sec)4. 子查询缺点，和联接查询一样，具体的效率要看索引的配置和MySQL的优化结果。mysql> select a.*,    ->  (select count(*) from tbl where col<=a.col) as rownum    -> from tbl a;+----+------+--------+| id | col  | rownum |+----+------+--------+|  1 |   26 |      1 ||  2 |   46 |      3 ||  3 |   35 |      2 ||  4 |   68 |      4 ||  5 |   93 |      6 ||  6 |   92 |      5 |+----+------+--------+6 rows in set (0.06 sec)做为一款开源的数据库系统，MySQL无疑是一个不做的产品。它的更新速度，文档维护都不逊于几大商业数据库产品。估计在下一个版本中，我们可以看到由MySQL自身实现的ROWNUM。----------------------------------------------------------------------------------------------------应用：同一个表中相邻的两条记录进行运算中的cpu_busy，cpu_total。  cpu_total=usr+nice+sys+idle+iowait+irq+softirq;  cpu_busy=cpu_total-idle;  计算cpu利用率：  cpu_usage=(cpu_busy2-cpu_busy1)/(cpu_total2 -cpu_total 1)  [相邻两行怎么计算差值？？]select usr+nice+sys+idle+iowait+irq+softirq ascpu_totalfrom os;select usr+nice+sys+iowait+irq+softirq as cpu_busyfrom os;思路:os表和os表进行自身的左连接操作，条件是表A的A.timestamp_+1等于表B的B.timestamp_。select  A.timestamp_,  A.cpu_total , A.cpu_busy, B.timestamp_ ,  B.cpu_total ,   B.cpu_busy, B.cpu_busy-A.cpu_busy ,B.cpu_total-A.cpu_total, (B.cpu_busy-A.cpu_busy)/(B.cpu_total-A.cpu_total) as cpu_usage from(select timestamp_,  usr+nice+sys+idle+iowait+irq+softirq as cpu_total ,usr+nice+sys+iowait+irq+softirq as cpu_busy from os ) A left join (select timestamp_,  usr+nice+sys+idle+iowait+irq+softirq as cpu_total ,usr+nice+sys+iowait+irq+softirq as cpu_busy from os ) Bon A.timestamp_+1=B.timestamp_;把一个表的某几项数据，插入到另一个表中。UPDATE  表A  INNER  JOIN   表B   ON  表A的字段=表B的字段(条件) SET 表A的等待修改项=表B的某个字段update  os  inner  join (select  timestamp_ from  ) D on  timestamp_=D.btimestamp_ set os.cpu_usage=D.cpu_usage;