SQL 行列倒置

SQL的的行列倒置已经不是新知识了，但在博主的技术咨询期间，仍发现其实有很多人并不了解这块，所以在此专门写一篇博客记录。本文将以Mysql为例，并以数据采集指标信息获取为例子。在下面的例子，你可以在sqlfiddle运行。

首先我们需要创建数据库Schema：

    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   value
February, 01 2015 12:00:00  disk    disk    20
February, 01 2015 12:15:00  disk    disk    30
February, 01 2015 12:20:00  disk    disk    25
February, 01 2015 12:00:00  disk    disk-all    20
February, 01 2015 12:20:00  disk    disk-all    30
February, 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                       0
disk-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 Chart
WHERE 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。

下面我们来分析下这句SQL，

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 Chart
WHERE 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                       0
disk                0                       30                      0                       0
disk                0                       0                       25                      0
disk-all            20                      0                       0                       0
disk-all            0                       0                       30                      0
disk-all            0                       0                       0                       25

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 Chart
WHERE 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：

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。
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;

破狼

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