用简单程序替换 MySQL 窗口函数

窗口函数是SQL2003标准才开始有的一系列SQL函数，用于应付一些复杂运算是比较方便。但是普遍使用的MySQL数据库对窗口函数支持得却很不好，直到最近的版本才开始有部分支持，这当然就让MySQL程序员很郁闷了。

实际操作中，我们可以在MySQL里用SQL拼出窗口函数功能，但是需要使用用户变量以及多个SELECT表达式从左到右依次计算的隐含规则。下面我们来看两个例子（为调试方便，我们直接用集算器作为测试环境）。

 

1、2016年1月销售额排名

	A
1	set @i1=0, @i2=0, @d1=null;
2	select @i1:=@i1+1 `row_number`, province, curr_sales, prev_sales,   @i2:=if(prev_sales=curr_sales,@i2,@i1) `rank` from (select province,  cast(@d1 as decimal(15,2)) as prev_sales, @d1:=sales as curr_sales from detail where yearmonth=201601 order by sales desc ) t1;
3	=connect(“mysql”)
4	>A3.execute(A1)
5	=A3.query@x(A2)

(1)A1中语句用于初始化用户变量；

(2)A2中语句先对销售额排倒序，然后每一行销售额与上一行销售额比较，若相等则排名不变，否则排名等于行号；

(3)A3连接数据库；

(4)A4执行初始化语句；

(5)A5执行查询语句并关闭数据库连接，返回结果。

执行后A5为需要的结果。

2、2016年1月和2月销售额按月分组百分比排名

	A
1	set @i1=null, @i2=0, @i3=0, @d1=null;
2	select curr_month, t1.province, curr_sales, sale_rank,   if(count>1, (sale_rank-1)/(count-1), 0) as `percent_rank` from (select prev_month, curr_month, province, @i2:=if(prev_month=curr_month,@i2+1,1) as `row_number`, @i3:=if(prev_month<>curr_month, 1, if(prev_sales=curr_sales, @i3, @i2)) as ‘sale_rank’, prev_sales, curr_sales from (select @i1 as prev_month, @i1:=yearmonth as curr_month, province, @d1 as prev_sales, @d1:=sales as curr_sales from (select * from detail where yearmonth in (201601,201602) order by yearmonth, sales desc ) t111 ) t11 ) t1 join (select yearmonth, province, count(*) count from detail where yearmonth in (201601, 201602) group by yearmonth ) t2 on t1.curr_month=t2.yearmonth;
3	=connect(“mysql”)
4	>A3.execute(A1)
5	=A3.query@x(A2)

(1)A1中语句用于初始化用户变量；

(2)A2中语句子查询t11求出上一行的月份和销售额，t1再求出本月行号与排名，t2算出每月的行数，最后t1与t2连接再利用公式[if(本月行数>1,(当前行的本月排名-1)/(本组行数-1),0)]求出百分比排号。

执行后A5为需要的结果。

通过上述两个例子，我们可以看到，为了实现窗口函数相应功能，SQL语句冗长、复杂而且可读性较差。另外，这里还使用了SELECT表达式从左到右依次计算的隐含规则，而这在MySQL参考手册是不推荐使用的，如果今后不能使用这一规则，那么写出来的SQL语句会更加复杂。譬如不使用这条隐含规则如何能取上一行的字段值呢？各位读者可以自行脑补。

 

值得庆幸的是，有了集算器及其特有的SPL语言，我们就大可不必这么麻烦了，MySQL只要使用最基本的SQL就行了，剩下的事由集算器来完成。

下面我们就来看看集算器的SPL语法是如何实现相应窗口函数的功能的。

1、SUM()、COUNT()、AVG()、MAX()、MIN()、VARIANCE

a)select province, sales, sum(sales) over() `sum`,

avg(sales) over() `avg`, max(sales) over() `max`,

min(sales) over() `min`, count(*) over() `count`

from detail

where yearmonth=201601

order by sales;

	A
1	=connect(“mysql”)
2	=A1.query@x(“select * from detail where yearmonth=201601 order by sales desc”)
3	=A2.sum(sales)
4	=A2.avg(sales)
5	=A2.max(sales)
6	=A2.min(sales)
7	=A2.count()
8	=A2.new(province, sales, A3:sum, A4:avg,A5:max,A6:min, A7:count)

(1)A3到A7依次对销售额求和、求平均、求最大、求最小及求总行数；

(2)A8构造序表，其中每一行都有本月销售额总和、平均值、最大值、最小值及总行数

执行后A8的结果如下:

这个例子很常规，毫无挑战性，只是小练一把，下面开始玩真的。

 

b)select yearmonth,province,sales,

sum(sales) over (partition by yearmonth) `sum`,

avg(sales) over (partition by yearmonth) `avg`,

max(sales) over (partition by yearmonth) `max`,

min(sales) over (partition by yearmonth) `min`,

count(*) over (partition by yearmonth) `count`

from detail

where yearmonth in (201601,201602) and sales>49500

order by yearmonth, sales desc;

	A
1	=connect(“mysql”)
2	=A1.query@x(“select * from detail where yearmonth in (201601,201602) and sales>49500 order by yearmonth,sales desc”)
3	=A2.groups(yearmonth;sum(sales):sum,avg(sales):avg,max(sales):max,min(sales):min, count(1):count)
4	=A2.switch(yearmonth,A3)
5	=A4.new(yearmonth.yearmonth:yearmonth,province,sales,yearmonth.sum:sum, yearmonth.avg:avg,yearmonth.max:max,yearmonth.min:min,yearmonth.count:count)

(1)A2中按月份分组并对销售额求和、求平均、求最大、求最小及每组行数；

(2)A4按月份将A2中yearmonth字段值转换成A3中相同月份的记录

执行后A5的结果如下。

2、VARIANCE()、STD()

a)select province, sales, variance(sales) over() `variance`, std(sales) over() `std`

from detail where yearmonth=201601;

	A
1	=connect(“mysql”)
2	=A1.query(“select * from detail where yearmonth=201601”)
3	=A2.variance(sales)
4	=sqrt(A3)
5	=A2.new(province,sales,A3:variance,A4:std)

(1)A3对销售额求方差。

(2)A4对A3求平方根即为标准差

执行后A5的结果如下。

b)select yearmonth, province, sales,

variance(sales) over(partition by yearmonth) `variance`,

std(sales) over(partition by yearmonth) `std`

from detail

where yearmonth in (201601, 201602);

	A
1	=connect(“mysql”)
2	=A1.query@x(“select * from detail where yearmonth in (201601,201602) order by yearmonth”)
3	=A2.group(yearmonth)
4	=A3.new(yearmonth:m,~.variance(sales):v, sqrt(v):v2)
5	=A2.switch(yearmonth, A4:m)
6	=A5.new(yearmonth.m:yearmonth, province, sales, yearmonth.v:variance, yearmonth.v2:std)

(1)A3按月份分组

(2)A4求每月销售额的方差

执行后A6的结果如下:

3、ROW_NUMBER()、RANK()、DENSE_RANK()、PERCENT_RANK()

a)select province, sales, row_number() over(order by sales desc) `row_number`,

rank() over (order by sales desc) `rank`,

dense_rank() over (order by sales desc) `dense_rank`,

percent_rank() over (order by sales desc) `percent_rank`

from detail

where yearmonth=201601;

	A
1	=connect(“mysql”)
2	=A1.query(“select * from detail where yearmonth=201601”)
3	=A2.sort(sales:-1)
4	=A2.count()
5	=A3.new(province,sales,#:row_number,rank(sales):rank,ranki(sales):dense_rank, if(A4>1,(rank-1)/(A4-1),0):percent_rank)

(1)A5中#表示当前行在A3中的序号

(2)百分比排名的公式=if(行数>1,(排名-1)/(行数-1))

执行后A5的结果如下:

b)select province, sales,

row_number() over(partition by yearmonth order by sales desc)

`row_number`,

rank() over (partition by yearmonth order by sales desc) `rank`,

dense_rank() over (partition by yearmonth order by sales desc)

`dense_rank`,

percent_rank() over (partition by yearmonth order by sales desc)

`percent_rank`

from detail

where yearmonth in (201601,201602);

	A
1	=connect(“mysql”)
2	=A1.query(“select * from detail where yearmonth in (201601,201602)”)
3	=A2.sort(yearmonth,sales:-1)
4	=A2.groups(yearmonth:m;count(1):count)
5	=A2.switch(yearmonth,A4:m)
6	=A3.new(yearmonth,province,sales,seq(yearmonth):row_number,rank(sales;yearmonth):rank, ranki(sales;yearmonth):dense_rank, if(yearmonth.count>1, (rank-1)/(yearmonth.count-1),0):percent_rank)

执行后A6的结果如下:

4、NTILE()

a)select province, sales, ntile(3) over() `ntile`

from detail

where yearmonth=201601;

	A
1	=connect(“mysql”)
2	=A1.query@x(“select * from detail where yearmonth=201601”)
3	=桶数=3
4	=A2.count()
5	=A2.new(province,sales,z(#,桶数,A4):ntile)

(1)A3里指明桶数为3

(2)A5中z(i,桶数,总行数)计算第i行所在桶号

执行后A9的结果如下:

b)select yearmonth, province, sales, ntile(3) over(partition by yearmonth) `ntile`

from detail

where yearmonth=201601 or( yearmonth=201602 and province!=’上海’);

	A
1	=connect(“mysql”)
2	=A1.query@x(“select * from detail where yearmonth=201601 or (yearmonth=201602 and province!=’上海’) order by yearmonth” )
3	=桶数=3
4	=A2.group(yearmonth:m;~.count():count)
5	=A2.switch(yearmonth,A4:m)
6	=A5.new(yearmonth.m:yearmonth,province,sales, z(seq(yearmonth), 桶数, yearmonth.count):ntile)

执行后A6的结果如下:

5、FIRST_VALUE()、LAST_VALUE()、NTH_VALUE()、LAG()、LEAD()

a)select province,sales,

first_value(sales) over(partition by yearmonth) `first_value`,

last_value(sales) over(partition by yearmonth) `last_value`,

nth_value(sales, 5) over(partition by yearmonth) `nth_value`,

lag(sales, 2) over(partition by yearmonth) `lag`,

lead(sales, 3) over(partition by yearmonth) `lead`

from detail

where yearmonth=201601;

	A
1	=connect(“mysql”)
2	=A1.query@x(“select * from detail where yearmonth=201601”)
3	=A2.new( province, sales, A2.m(1).sales:first_value,A2.m(-1).sales:last_value, A2.m(5).sales:nth_value, ~[-2].sales:lag,~[3].sales:lead)

(1)Am(i)取A2中第i条记录，越界返回null，负数则从后往前数第abs(i)条记录，不能使用A2(i)，因为A2(i)越界会报错

执行后A3的结果如下:

b)select yearmonth,province,sales,

first_value(sales) over(partition by yearmonth) `first_value`,

last_value(sales) over(partition by yearmonth) `last_value`,

nth_value(sales, 5) over(partition by yearmonth) `nth_value`,

lag(sales, 2) over(partition by yearmonth) `lag`,

lead(sales, 3) over(partition by yearmonth) `lead`

from detail

where yearmonth=201601 or (yearmonth=201602 and sales>50000);

	A
1	=connect(“mysql”)
2	=A1.query@x(“select * from detail where yearmonth=201601 or (yearmonth=201602 and sales>50000) order by yearmonth”)
3	=A2.group(yearmonth:m;~.count():count,~.m(1).sales:first_value, ~.m(-1).sales:last_value,~.m(5).sales:nth_value)
4	=A2.switch(yearmonth, A3:m)
5	=A2.new(yearmonth.m:yearmonth, province, sales, yearmonth.first_value:first_value,yearmonth.last_value:last_value,yearmonth.nth_value:nth_value, (seq=seq(yearmonth),if(seq>2,~[-2].sales,null)):lag,if(yearmonth.count-seq>=3,~[3].sales,null):lead)

(1)A5中，seq(yearmonth)尽可能不要在if函数中使用，因为seq函数是在对A2中记录循环过程中累加的，导致seq函数少执行1次就少累加1。

(2)A5中，前面的表达式用seq=seq(yearmonth)对变量seq赋值，这样后续表达式就可以引用变量seq。

执行后A5的结果如下:

6、CUME_DIST()

a)select province,sales, cume_dist() over(order by sales) `cume_dist`

from detail

where yearmonth=201601;

	A
1	=connect(“mysql”)
2	=A1.query@x(“select * from detail where yearmonth=201601 order by sales desc”)
3	=A2.count()
4	=A2.new(province,sales,(A3-rank(sales)+1)/A3:cume_dist)
5	=A4.rvs()

(1)CUME_DIST() over (order by sales)求销售额从小到大的累积概率分布，公式为(小于等于当前销售额的行数/总行数)

(2)小于等于当前销售额的行数=总行数-当前销售额从大到小的排名+1

(3)A2必须按销售额从大到小排序

(4)A5数据倒排

执行后A5的结果如下:

b)select yearmonth, province,sales,

cume_dist() over(partition by yearmonth order by sales) `cume_dist`

from detail

where yearmonth in (201601,201602);

	A
1	=connect(“mysql”)
2	=A1.query@x(“select * from detail where yearmonth in (201601,201602) order by yearmonth desc,sales desc”)
3	=A2.groups(yearmonth:m;count(1):count)
4	=A2.switch(yearmonth,A3:m)
5	=A2.new(yearmonth.m:yearmonth,province,sales,(yearmonth.count-rank(sales;yearmonth)+1)/yearmonth.count:cume_dist)
6	=A5.rvs()

(1)对应于最后的倒排，A2中按月份从大到小排序

执行后A6的结果如下:

看完十多个例子，有没有觉得集算器代码实现so easy？！而且，由于集算器可以对单元格进行分步计算，我们可以按照自然的思路逐步查看查询结果，从而更加简便、直观地完善整个查询脚本。赶紧用起来吧，你会发现更多又方便又强大的功能！