樞紐轉換

使用 SQL 分析商業資料

Michel Semaan

Data Scientist

什麼是樞紐轉換？

樞紐轉換（Pivoting）：繞著樞紐欄位旋轉資料表，將某欄位的值轉成多個欄位
- 將「長表」轉為「寬表」

優點

在保留資料下，調整資料表的外觀
將堆疊資料改為橫向呈現，通常比縱向更易讀

轉換前

meal_id  delivr_month  count_orders
-------  ------------  ------------
0        2018-06-01    39          
0        2018-07-01    47          
1        2018-06-01    25          
1        2018-07-01    55

轉換後

meal_id  2018-06-01  2018-07-01
-------  ----------  ----------
0        39          47        
1        25          55

以 delivr_month 為樞紐轉換

轉換前的資料表

SELECT
  meal_id,
  DATE_TRUNC('month', order_date) :: DATE AS delivr_month,
  COUNT(DISTINCT orders) :: INT AS revenue
FROM meals
JOIN orders ON meals.meal_id = orders.meal_id
WHERE meals.meal_id IN (0, 1)
  AND order_date < '2018-08-01'
GROUP BY meal_id, delivr_month
ORDER BY meal_id, delivr_month;

CROSSTAB()

CREATE EXTENSION IF NOT EXISTS tablefunc;

CREATE EXTENSION 類似於 Python 的 import

SELECT * FROM CROSSTAB($$
  TEXT source_sql
$$)

AS ct (column_1 DATA_TYPE_1,
       column_2 DATA_TYPE_2,
       ...,
       column_n DATA_TYPE_N)
;

使用 CROSSTAB()

CREATE EXTENSION IF NOT EXISTS tablefunc;


SELECT * FROM CROSSTAB($$
  SELECT
    meal_id,
    DATE_TRUNC('month', order_date) :: DATE AS delivr_month,
    COUNT(DISTINCT order_id) :: INT AS orders
  FROM orders
  WHERE meal_id IN (0, 1)
    AND order_date < '2018-08-01'
  GROUP BY meal_id, delivr_month
  ORDER BY meal_id, delivr_month $$)

  AS ct (meal_id INT,
         "2018-06-01" INT,
         "2018-07-01" INT)
ORDER BY meal_id ASC;

轉換前的表

meal_id  delivr_month  count_orders
-------  ------------  ------------
0        2018-06-01    39          
0        2018-07-01    47          
1        2018-06-01    25          
1        2018-07-01    55

轉換後的表

meal_id  2018-06-01  2018-07-01
-------  ----------  ----------
0        39          47        
1        25          55

以 delivr_month 為樞紐轉換

樞紐轉換

使用 SQL 分析商業資料