| MySQL分析函数实现
还好MySQL8.0已经实现了与Oracle相同的分析函数。
1. 实现rownum
SET @rn:=0;
SELECT @rn:=@rn+1 AS rownum ,e.* FROM emp e;
或者写成：
SELECT @rn:=@rn + 1 AS rownum ,e.* FROM emp e ,(SELECT @rn:=0) c
2. 各种分析函数写法 (MySQL实现分析语句时可能遇到的各种计算问题)
2.1 sum() 实现
--SQL 执行顺序 ,FROM ,JOIN ,WHERE ,GROUP BY,HAVING ,ORDER BY ,SELECT,
在Oracle中分页语句的原始语句如下：
SELECT E.*, SUM(SAL) OVER(PARTITION BY DEPTNO) AS COUNTOVER FROM EMP E;
SELECT E.*,
(SELECT SUMOVER
FROM (SELECT DEPTNO, SUM(SAL) AS SUMOVER
FROM EMP E1
GROUP BY DEPTNO) X
WHERE X.DEPTNO = E.DEPTNO) AS COUNTOVER
FROM EMP E
ORDER BY DEPTNO;
Mysql中也是这么实现的：
SELECT E.*,
(SELECT SUMOVER
FROM (SELECT DEPTNO, SUM(SAL) AS SUMOVER
FROM emp E1
GROUP BY DEPTNO) X
WHERE X.DEPTNO = E.DEPTNO) AS COUNTOVER
FROM emp E
ORDER BY DEPTNO;
2.2 row_number () 实现
select
e.* ,row_number() over(partition by deptno order by empno) as ROW_NUMBER from emp e;
我们的默认规则是在from后初始化变量。
SELECT E.*,
IF(@DEPTNO = DEPTNO, @RN := @RN + 1, @RN := 1) AS ROW_NUMBER,
@DEPTNO := DEPTNO AS VAR1
FROM EMP E, (SELECT @DEPTNO := '', @RN := 0) C
ORDER BY DEPTNO;
SELECT E.*,
IF(@DEPTNO = DEPTNO, @RN := @RN + 1, @RN := 1) AS ROW_NUMBER,
@DEPTNO := DEPTNO AS VAR1
FROM EMP E, (SELECT @DEPTNO := '', @RN := 0) C
ORDER BY DEPTNO;
这个语句首先执行order by
2.3 求每个人员占他所在部门总工资的百分比
在Oracle中实现：
SELECT E.*,
TRUNC(SAL / SUM(SAL) OVER(PARTITION BY DEPTNO), 3) AS SALPERCENT
FROM EMP E
ORDER BY DEPTNO;
SELECT E.*,
SAL / (SELECT SUMOVER
FROM (SELECT DEPTNO, SUM(SAL) AS SUMOVER
FROM emp E1
GROUP BY DEPTNO) X
WHERE X.DEPTNO = E.DEPTNO) AS SalPercent
FROM emp E
ORDER BY DEPTNO;
2.4 求各个部门的总共工资
Oracle:
SELECT e.* ,SUM(sal) OVER(PARTITION BY deptno) FROM emp e;
MySQL:
SELECT A.*,
ROUND(CAST(IF(@DEPTNO = DEPTNO, @MAX := @MAX, @MAX := SUMOVER) AS CHAR ),0) AS SUMOVER2,
@DEPTNO := DEPTNO AS VAR2
FROM (SELECT E.*,
IF(@DEPTNO = DEPTNO, @SUM := @SUM + SAL, @SUM := SAL) AS SUMOVER,
@DEPTNO := DEPTNO AS VAR1
FROM emp E, (SELECT @DEPTNO := '', @SUM := 0, @MAX := 0) C
ORDER BY DEPTNO) A
ORDER BY DEPTNO, SUMOVER DESC;
子查询的功能实现如下：
下面是这个语句的结果
2.5 拿部门第二的工资的人
首先我们拿第二名的，用Oracle很好实现，不论是第一还是第二。
SELECT *
FROM (SELECT E.*,
ROW_NUMBER() OVER(PARTITION BY DEPTNO ORDER BY SAL DESC) AS RN
FROM EMP E)
WHERE RN = 2;
Mysql中第一这么实现：
在5.6版本，sql_mode非only_full_group_by的情况，我们可以使用如下方式实现
set global sql_mode ='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';
SELECT * FROM (SELECT e.* FROM emp e ORDER BY deptno,sal ) a GROUP BY deptno;
在SQL_MODE非only_full_group_by时，MySQL中的group by是只取第一行的，下面我们看取第二行的SQL。
SELECT *
FROM (SELECT E.*,
IF(@DEPTNO = DEPTNO, @RN := @RN + 1, @RN := 1) AS RN,
@DEPTNO := DEPTNO
FROM EMP E, (SELECT @RN := 0, @DEPTNO := 0) C
ORDER BY DEPTNO, SAL DESC) X
WHERE X.RN = 2;
2.6 dense_rank()
dense_rank函数返回一个唯一的值，除非当碰到相同数据时，此时所有相同数据的排名都一样。
SELECT empno,
ename,
sal,
deptno,
rank() OVER(PARTITION BY deptno ORDER BY sal desc) as rank,
dense_rank() OVER(PARTITION BY deptno ORDER BY sal desc) as dense_rank
FROM emp e;
MySQL的写法：
select
empno,ename,sal,deptno,
if(@deptno = deptno,if(@sal=sal,@rn:=@rn,@rn3:=@rn3+1),@rn:=1) as "RANK() OVER",
if(@sal =sal,@rn2:=@rn2 ,if(@deptno = deptno,@rn2:=@rn2+1,@rn2:=1)) as "DENSE_RANK() OVER",
if(@deptno = deptno,@rn:=@rn+1,@rn:=1) as "ROW_NUMBER() OVER"
, @deptno:=deptno,@sal:=sal
from
(select empno,ename,sal,deptno from emp a ,(select @rn:=1,@deptno:=0,@rn2:=0,@rn3:=0,@sal:=0,@i:=0) b order by deptno,sal desc) c;
2.7 连续获得冠军的有哪些
--请写出一条SQL语句，查询出在此期间连续获得冠军的有哪些，其连续的年份的起止时间是多少，结果如下：
create table nba as
SELECT '公牛' AS TEAM, '1991' AS Y FROM DUAL UNION ALL
SELECT '公牛' AS TEAM, '1992' AS Y FROM DUAL UNION ALL
SELECT '公牛' AS TEAM, '1993' AS Y FROM DUAL UNION ALL
SELECT '活塞' AS TEAM, '1990' AS Y FROM DUAL UNION ALL
SELECT '火箭' AS TEAM, '1994' AS Y FROM DUAL UNION ALL
SELECT '火箭' AS TEAM, '1995' AS Y FROM DUAL UNION ALL
SELECT '公牛' AS TEAM, '1996' AS Y FROM DUAL UNION ALL
SELECT '公牛' AS TEAM, '1997' AS Y FROM DUAL UNION ALL
SELECT '公牛' AS TEAM, '1998' AS Y FROM DUAL UNION ALL
SELECT '马刺' AS TEAM, '1999' AS Y FROM DUAL UNION ALL
SELECT '湖人' AS TEAM, '2000' AS Y FROM DUAL UNION ALL
SELECT '湖人' AS TEAM, '2001' AS Y FROM DUAL UNION ALL
SELECT '湖人' AS TEAM, '2002' AS Y FROM DUAL UNION ALL
SELECT '马刺' AS TEAM, '2003' AS Y FROM DUAL UNION ALL
SELECT '活塞' AS TEAM, '2004' AS Y FROM DUAL UNION ALL
SELECT '马刺' AS TEAM, '2005' AS Y FROM DUAL UNION ALL
SELECT '热火' AS TEAM, '2006' AS Y FROM DUAL UNION ALL
SELECT '马刺' AS TEAM, '2007' AS Y FROM DUAL UNION ALL
SELECT '凯尔特人' AS TEAM, '2008' AS Y FROM DUAL UNION ALL
SELECT '湖人' AS TEAM, '2009' AS Y FROM DUAL UNION ALL
SELECT '湖人' AS TEAM, '2010' AS Y FROM DUAL;
Oracle实现：
SELECT TEAM, MIN(Y), MAX(Y)
FROM (SELECT E.*,
ROWNUM,
ROW_NUMBER() OVER(PARTITION BY TEAM ORDER BY Y) AS RN,
ROWNUM - ROW_NUMBER() OVER(PARTITION BY TEAM ORDER BY Y) AS DIFF
FROM NBA E
ORDER BY Y)
GROUP BY TEAM, DIFF
HAVING MIN(Y) != MAX(Y)
ORDER BY 2;
MySQL实现：
SELECT TEAM, MIN(Y), MAX(Y)
FROM (SELECT TEAM,
Y,
IF(@TEAM = TEAM, @RN := @RN + 1, @RN := 1) AS RWN,
@RN1 := @RN1 + 1 AS RN,
@TEAM := TEAM
FROM nba N, (SELECT @RN := 0, @TEAM := '', @RN1 := '') C) A
GROUP BY RN - RWN
HAVING MIN(Y) != MAX(Y)
ORDER BY 2
| UDF插件
Userdefined Function，用户定义函数。我们知道，MySQL本身支持很多内建的函数，此外还可以通过创建存储方法来定义函数。UDF为用户提供了一种更高效的方式来创建函数。
UDF与普通函数类似，有参数，也有输出。分为两种类型：单次调用型和聚集函数。前者能够针对每一行数据进行处理，后者则用于处理Group By这样的情况。
UDF自定义函数，在MySQL basedir/include
[root@test12c include]# pwd
/usr/local/mysql/include
[root@test12c include]# cat rownum.c
#include <my_global.h>
#include <my_sys.h>
#if defined(MYSQL_SERVER)
#include <m_string.h> /* To get strmov() */
#else
/* when compiled as standalone */
#include <string.h>
#define strmov(a,b) stpcpy(a,b)
#endif
#include <mysql.h>
#include <ctype.h>
/*
gcc -fPIC -Wall -I/usr/local/mysql/include -I. -shared rownum.c -o rownum.so
DROP FUNCTION IF EXISTS rownum;
CREATE FUNCTION rownum RETURNS INTEGER SONAME 'rownum.so';
*/
C_MODE_START;
my_bool rownum_init(UDF_INIT *initid, UDF_ARGS *args, char *message);
void rownum_deinit(UDF_INIT *initid);
chong rownum(UDF_INIT *initid, UDF_ARGS *args, char *is_null,char *error);
C_MODE_END;
/*
Simple example of how to get a sequences starting from the first argument
or 1 if no arguments have been given
*/
my_bool rownum_init(UDF_INIT *initid, UDF_ARGS *args, char *message)
{
if (args->arg_count > 1)
{
strmov(message,"This function takes none or 1 argument");
return 1;
}
if (args->arg_count)
args->arg_type[0]= INT_RESULT; /* Force argument to int */
if (!(initid->ptr=(char*) malloc(sizeof(chong))))
{
strmov(message,"Couldn't allocate memory");
return 1;
}
memset(initid->ptr, 0, sizeof(chong));
initid->const_item=0;
return 0;
}
void rownum_deinit(UDF_INIT *initid)
{
if (initid->ptr)
free(initid->ptr);
}
chong rownum(UDF_INIT *initid __attribute__((unused)), UDF_ARGS *args,char *is_null __attribute__((unused)),char *error __attribute__((unused)))
{
uchong val=0;
if (args->arg_count)
val= *((chong*) args->args[0]);
return ++*((chong*) initid->ptr) + val;
}
生成动态链接库
gcc rownum.c -fPIC -shared -o ../lib/plugin/rownum.so
| 作者简介
姚崇·沃趣科技高级数据库技术专家
熟悉Oracle、MySQL数据库内部机制，丰富的Oracle、MySQL故障诊断、性能调优、数据库备份恢复、复制、高可用方案及迁移经验。
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作者：woqutechteam
来源：CSDN
原文：https://blog.csdn.net/woqutechteam/article/details/83501038
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