SQL 行列倒置

 CREATE TABLE Chart        (`createTime` DateTime, `kpi` varchar(30), `field` varchar(30), `value` double);    INSERT INTO Chart        (`createTime`,`kpi`, `field`, `value`)    VALUES        ("2015-02-01 12:00:00", 'disk', 'disk', 20),        ("2015-02-01 12:15:00", 'disk', 'disk', 30),        ("2015-02-01 12:20:00", 'disk', 'disk', 25),        ("2015-02-01 12:30:00", 'disk', 'disk', 25),        ("2015-02-01 12:35:00", 'disk', 'disk', 25),        ("2015-02-01 12:40:00", 'disk', 'disk', 25),        ("2015-02-01 12:00:00", 'disk', 'disk-all', 20),        ("2015-02-01 12:20:00", 'disk', 'disk-all', 30),        ("2015-02-01 12:25:00", 'disk', 'disk-all', 25),        ("2015-02-01 12:30:00", 'disk', 'disk-all', 25),        ("2015-02-01 12:35:00", 'disk', 'disk-all', 25),        ("2015-02-01 12:40:00", 'disk', 'disk-all', 25),        ("2015-02-01 12:40:00", 'cpu', 'cpu-all', 25),        ("2015-02-01 12:40:00", 'cpu', 'cpu', 25)    ;

在这里字段分别代表：createTime = 数据采集时间，kpi = 数据采集指标，field = 作为指标的小类(一个kpi可以包含多个field)，value = 采集的数据

当我们创建好了数据结构，下面因为我们希望获取出所有的 固定时间范围内的特定kpi的数据，注意因为可能一个kpi中的多个field，但是某些field漏采了部分时间的数据，所以这里我们需要补充异常点0. 并由于EChart这类图表库，希望我们输入的是横轴和纵轴为两个独立的数组对象表示。所以我们需要如下：

option = {    ....    xAxis : [        {            type : 'category',            boundaryGap : false,            data : ['周一','周二','周三','周四','周五','周六','周日']        }    ],    yAxis : [        {            type : 'value',            axisLabel : {                formatter: '{value} °C'            }        }    ],    series : [        {            ....            data:[11, 11, 15, 13, 12, 13, 10]        },        {           ....            data:[11, 11, 15, 13, 12, 13, 10]        }    ]};

取出横轴比较容易，如下：

SELECT createTime,kpi, field, value FROM Chart WHERE kpi = 'disk' and (createTime BETWEEN '2015-02-01 12:00:00' AND '2015-02-01 12:25:00');

但是纵轴如果我们以同样方式取出，可能存在需要我们自动程序补值，并且需要保证每项数据和横轴对应，所以我们的程序处理会比较复杂，如下：

SELECT createTime,kpi, field, value FROM Chart WHERE kpi = 'disk' and (createTime BETWEEN '2015-02-01 12:00:00' AND '2015-02-01 12:25:00');

结果为：

createTime  kpi field   valueFebruary, 01 2015 12:00:00  disk    disk    20February, 01 2015 12:15:00  disk    disk    30February, 01 2015 12:20:00  disk    disk    25February, 01 2015 12:00:00  disk    disk-all    20February, 01 2015 12:20:00  disk    disk-all    30February, 01 2015 12:25:00  disk    disk-all    25

有没有其他方案更佳的呢？当然那就是本文要说的sql的倒置，如果我们能够把返回数据转换为如下：

field   ‘2015-02-01 12:00:00’   ‘2015-02-01 12:15:00’   ‘2015-02-01 12:20:00’   ‘2015-02-01 12:25:00’disk         20                            30                     25                       0disk-all     20                             0                     30                       25

那么程序就很好处理了。在上面我们已经能够取出所有的横轴数据并排序，接下来我们将可以很简单的做到行列倒置：如下：

SELECT field,SUM(IF(createTime = '2015-02-01 12:00:00', value, 0)) as '2015-02-01 12:00:00',SUM(IF(createTime = '2015-02-01 12:15:00', value, 0)) as '2015-02-01 12:15:00',SUM(IF(createTime = '2015-02-01 12:20:00', value, 0)) as '2015-02-01 12:20:00',SUM(IF(createTime = '2015-02-01 12:25:00', value, 0)) as '2015-02-01 12:25:00' FROM ChartWHERE kpi = 'disk' and (createTime BETWEEN '2015-02-01 12:00:00' AND '2015-02-01 12:25:00')GROUP BY field

这样返回数据满足我们的需求了。

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下面我们来分析下这句SQL，

1. 首先我们利用‘IF(createTime = ‘2015-02-01 12:00:00’, value, 0)’来处理插值，并对每行数据转为以时间为列数据,并可以利用IF来补’0‘，将会如下：

SQL：

SELECT field,IF(createTime = '2015-02-01 12:00:00', value, 0) as '2015-02-01 12:00:00',IF(createTime = '2015-02-01 12:15:00', value, 0) as '2015-02-01 12:15:00',IF(createTime = '2015-02-01 12:20:00', value, 0) as '2015-02-01 12:20:00',IF(createTime = '2015-02-01 12:25:00', value, 0) as '2015-02-01 12:25:00' FROM ChartWHERE kpi = 'disk' and (createTime BETWEEN '2015-02-01 12:00:00' AND '2015-02-01 12:25:00');

结果为：

field   ‘2015-02-01 12:00:00’   ‘2015-02-01 12:15:00’   ‘2015-02-01 12:20:00’   ‘2015-02-01 12:25:00’disk               20                       0                       0                       0disk                0                       30                      0                       0disk                0                       0                       25                      0disk-all            20                      0                       0                       0disk-all            0                       0                       30                      0disk-all            0                       0                       0                       25

1. 这下我们就可以利用sql的聚合函数sum和group by来聚合数据行：

SQL:

SELECT field,SUM(IF(createTime = '2015-02-01 12:00:00', value, 0)) as '2015-02-01 12:00:00',SUM(IF(createTime = '2015-02-01 12:15:00', value, 0)) as '2015-02-01 12:15:00',SUM(IF(createTime = '2015-02-01 12:20:00', value, 0)) as '2015-02-01 12:20:00',SUM(IF(createTime = '2015-02-01 12:25:00', value, 0)) as '2015-02-01 12:25:00' FROM ChartWHERE kpi = 'disk' and (createTime BETWEEN '2015-02-01 12:00:00' AND '2015-02-01 12:25:00')GROUP BY field

效果如上。

对于sql行列转置可以简述为分为两部分：

1. 利用条件逻辑(mysql： IF， sql server： case … when(sql server 2005开始支持数据透视表pivot) ..)将 需要倒置的数据变为列。
2. 利用聚合函数(sum、max、min…)group by 合并数据。这里需要注意max、min需要注意数据的边界，如存在负数且默认值采用0，那么max就会存在问题，所以一般sum是最安全的(任何数加0都不会改变结果)；但对于特定场景max、min也是安全方案。

我们也可以将上面两次请求合并为一次，这就需要mysql的动态拼接，如下：

SELECT @time_sql := group_concat("SUM(IF(createTime = '", t.createTime, "', value, 0)) AS '" , t.createTime, "'")  FROM ( SELECT DISTINCT createTime FROM Chart ORDER BY createTime) AS t; set @v_sql = CONCAT("SELECT field", IF(ISNULL(@time_sql) , " ", CONCAT(", ", @time_sql)) ," FROM Chart GROUP BY field");prepare stmt from @v_sql; EXECUTE stmt;   deallocate prepare stmt;