为什么这个 crosstab() 查询 return 重复键？

Question

我有以下 table 叫 sample_events:

 Column | Type
--------+-----
 title  | text
 date   | date

具有值：

 title |     date
-------+------------
 ev1   | 2017-01-01
 ev2   | 2017-01-03
 ev3   | 2017-01-02
 ev4   | 2017-12-10
 ev5   | 2017-12-11
 ev6   | 2017-07-28

为了创建一个枢轴 table，其中包含每个唯一年份中每月的事件数，我使用了 crosstab(text source_sql, text category_sql):

形式的交叉表函数

SELECT * FROM crosstab (
   'SELECT extract(year from date) AS year,
        extract(month from date) AS month, count(*)
    FROM sample_events
    GROUP BY year, month'
,
   'SELECT * FROM generate_series(1, 12)'
) AS (
    year int, jan int, feb int, mar int,
    apr int, may int, jun int, jul int,
    aug int, sep int, oct int, nov int, dec int
) ORDER BY year;

结果如下，符合预期：

 year | jan | feb | mar | apr | may | jun | jul | aug | sep | oct | nov | dec
------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----
 2017 |   3 |     |     |     |     |     |   1 |     |     |     |     |   2

现在，我想创建一个数据透视表 table，其中包含一年中每个唯一星期中每周中每天的事件数。我尝试了以下查询：

SELECT * FROM crosstab (
   'SELECT extract(week from date) AS week,
        extract(dow from date) AS day_of_week, count(*)
    FROM sample_events
    GROUP BY week, day_of_week'
,
   'SELECT * FROM generate_series(0, 6)'
) AS (
    week int, sun int, mon int, tue int,
    wed int, thu int, fri int, sat int
) ORDER BY week;

结果与预期不符：

 week | sun | mon | tue | wed | thu | fri | sat 
------+-----+-----+-----+-----+-----+-----+-----
    1 |     |     |   1 |     |     |     |    
    1 |     |   1 |     |     |     |     |    
   30 |     |     |     |     |     |   1 |    
   49 |   1 |     |     |     |     |     |    
   50 |     |   1 |     |     |     |     |    
   52 |   1 |     |     |     |     |     |

所有六个事件都在那里，但无论出于何种原因，都有重复的周值。我希望结果是这样的：

 week | sun | mon | tue | wed | thu | fri | sat 
------+-----+-----+-----+-----+-----+-----+-----
    1 |     |   1 |   1 |     |     |     |    
   30 |     |     |     |     |     |   1 |    
   49 |   1 |     |     |     |     |     |    
   50 |     |   1 |     |     |     |     |    
   52 |   1 |     |     |     |     |     |

问题

1) 为什么后一个查询的结果包含重复的键值，而前一个查询没有？

2) 如何创建具有唯一周值的枢轴 table？

Answer 1

crosstab() 需要有序输入。您需要在输入中添加ORDER BY：

SELECT * FROM crosstab (
   'SELECT extract(week from date)::int AS week
         , extract(dow  from date)::int AS day_of_week
         , count(*)::int
    FROM   sample_events
    GROUP  BY week, day_of_week
    ORDER  BY week, day_of_week'
 ,  'SELECT generate_series(0, 6)'
) AS (
    week int, sun int, mon int, tue int,
    wed int, thu int, fri int, sat int
);

或者只是 ORDER BY week.

严格来说，同一个key的值（例子中的week）需要分组（按顺序来）。钥匙不必订购。但实现这一点的最简单和最便宜的方法是 ORDER BY（另外对键进行排序）。

或简称：

SELECT * FROM crosstab (
   'SELECT extract(week from date)::int
         , extract(dow  from date)::int
         , count(*)::int
    FROM   sample_events
    GROUP  BY 1, 2
    ORDER  BY 1, 2'  -- or just ORDER BY 1
 ,  'SELECT generate_series(0, 6)'
) AS ...

您的第一个带有月份的示例恰好有效，因为输入数据按顺序排列月份。但是，如果 table 中行的物理顺序发生变化（VACUUM、UPDATE、...），这可能会随时中断。您永远不能依赖关系 table.

中行的物理顺序

更多解释：

PostgreSQL Crosstab Query

为什么这个 crosstab() 查询 return 重复键？

Why does this crosstab() query return duplicate keys?

postgresql

date

crosstab