MySQL-with rollup函数运用 _20160930

在博客里http://www.cnblogs.com/Mr-Cxy/p/5898839.html提到了行转列，

如果想在下面这个表下面添加一行 总计 数据行SQL代码怎么实现 并且根据9月金额进行城市降序 总计置于底部呢

MySQL提供了 group by with rollup 函数进行group by 字段的汇总

但是order by 互斥的不能同时用

第一步还是是先计算各城市每个月的金额

SELECT b.城市,SUM(IF(b.年月=201607,b.金额,NULL)) AS 7月金额,SUM(IF(b.年月=201608,b.金额,NULL)) AS 8月金额,SUM(IF(b.年月=201609,b.金额,NULL)) AS 9月金额
FROM (
    SELECT city AS 城市,DATE_FORMAT(order_time,"%Y%m") AS 年月,SUM(pay_money) AS 金额
    FROM test_a03order AS a
    GROUP BY city,DATE_FORMAT(order_time,"%Y%m")
) AS b
GROUP BY b.城市

第二步我们先用group by with rollup 函数添加针对字段的汇总

SELECT b.城市,SUM(IF(b.年月=201607,b.金额,NULL)) AS 7月金额,SUM(IF(b.年月=201608,b.金额,NULL)) AS 8月金额,SUM(IF(b.年月=201609,b.金额,NULL)) AS 9月金额
FROM (
    SELECT city AS 城市,DATE_FORMAT(order_time,"%Y%m") AS 年月,SUM(pay_money) AS 金额
    FROM test_a03order AS a
    GROUP BY city,DATE_FORMAT(order_time,"%Y%m")
) AS b
GROUP BY b.城市 WITH ROLLUP

这个记录没有出现总计两个字，怎么实现呢 继续修改代码 ifnull()函数

第三 添加总计字样(有坑) ifnull()函数是将空字段另外命名

SELECT IFNULL(b.城市,"总计") AS 城市,SUM(IF(b.年月=201607,b.金额,NULL)) AS 7月金额,SUM(IF(b.年月=201608,b.金额,NULL)) AS 8月金额,SUM(IF(b.年月=201609,b.金额,NULL)) AS 9月金额
FROM (
    SELECT city AS 城市,DATE_FORMAT(order_time,"%Y%m") AS 年月,SUM(pay_money) AS 金额
    FROM test_a03order AS a
    GROUP BY city,DATE_FORMAT(order_time,"%Y%m")
) AS b
GROUP BY b.城市 WITH ROLLUP

第四 摆脱掉坑 

为什么说有坑呢 如果ifnull()函数放在上面代码位置 看似实现了总计的字样  ifnull()是针对用了with rollup 函数总计这个位置出现空字段时候修改它为总计字样的,

如果城市这一列里本身就含有没有命名的城市呢 这一列就会出现多个总计字样 因此我们需要将ifnull()函数放到嵌套的子表里 将空字段在子表里面就预先改成别名（不能是总计字样）

因此 使用2个ifnull()函数就不会有这样的问题 结果是一样的 

SELECT IFNULL(b.城市,"总计") AS 城市,SUM(IF(b.年月=201607,b.金额,NULL)) AS 7月金额,SUM(IF(b.年月=201608,b.金额,NULL)) AS 8月金额,SUM(IF(b.年月=201609,b.金额,NULL)) AS 9月金额
FROM (
    SELECT IFNULL(city,'空城市') AS 城市,DATE_FORMAT(order_time,"%Y%m") AS 年月,SUM(pay_money) AS 金额
    FROM test_a03order AS a
    GROUP BY city,DATE_FORMAT(order_time,"%Y%m")
) AS b
GROUP BY b.城市 WITH ROLLUP

  第五 排序 

在这篇博客http://www.cnblogs.com/Mr-Cxy/p/5910291.html 提到了order by field()自定义排序函数 如下图红框所示有这样一个默认的自定义排序规则

因此 如果想实现 根据9月金额进行城市降序 总计置于底部 效果 可以把上面代码当成一个子表嵌套 结合 order by field()自定义函数实现 

SELECT c.*
FROM (
    SELECT IFNULL(b.城市,"总计") AS 城市,SUM(IF(b.年月=201607,b.金额,NULL)) AS 7月金额,SUM(IF(b.年月=201608,b.金额,NULL)) AS 8月金额,SUM(IF(b.年月=201609,b.金额,NULL)) AS 9月金额
    FROM (
        SELECT IFNULL(city,'空城市') AS 城市,DATE_FORMAT(order_time,"%Y%m") AS 年月,SUM(pay_money) AS 金额
        FROM test_a03order AS a
        GROUP BY city,DATE_FORMAT(order_time,"%Y%m")
    ) AS b
    GROUP BY b.城市 WITH ROLLUP
) AS c
ORDER BY FIELD(城市,'总计'),c.9月金额 DESC