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PostgreSQL 集計統計とウィンドウ関数

Michel Semaan

Data Scientist

4 つの関数

相対

LAG(column, n) は現在行の n 行前の column の値を返す
LEAD(column, n) は現在行の n 行後の column の値を返す

絶対

FIRST_VALUE(column) はテーブルまたはパーティションの先頭の値を返す
LAST_VALUE(column) はテーブルまたはパーティションの最後の値を返す

LEAD

クエリ

WITH Hosts AS (
  SELECT DISTINCT Year, City
  FROM Summer_Medals)

SELECT
  Year, City,
  LEAD(City, 1) OVER (ORDER BY Year ASC)
    AS Next_City,
  LEAD(City, 2) OVER (ORDER BY Year ASC)
    AS After_Next_City
FROM Hosts
ORDER BY Year ASC;

結果

| Year | City      | Next_City | After_Next_City |
|------|-----------|-----------|-----------------|
| 1896 | Athens    | Paris     | St Louis        |
| 1900 | Paris     | St Louis  | London          |
| 1904 | St Louis  | London    | Stockholm       |
| 1908 | London    | Stockholm | Antwerp         |
| 1912 | Stockholm | Antwerp   | Paris           |
| ...  | ...       | ...       | ...             |

FIRST_VALUE と LAST_VALUE

クエリ

SELECT
  Year, City,
  FIRST_VALUE(City) OVER
    (ORDER BY Year ASC) AS First_City,
  LAST_VALUE(City) OVER (
   ORDER BY Year ASC
   RANGE BETWEEN
     UNBOUNDED PRECEDING AND
     UNBOUNDED FOLLOWING
  ) AS Last_City
FROM Hosts
ORDER BY Year ASC;

結果

| Year | City      | First_City | Last_City       |
|------|-----------|------------|-----------------|
| 1896 | Athens    | Athens     | London          |
| 1900 | Paris     | Athens     | London          |
| 1904 | St Louis  | Athens     | London          |
| 1908 | London    | Athens     | London          |
| 1912 | Stockholm | Athens     | London          |

既定では、ウィンドウはテーブル/パーティションの先頭から現在行まで
RANGE BETWEEN ... でウィンドウをテーブル/パーティションの末尾まで拡張

LEAD のパーティション化

PARTITION BY なしの LEAD(Champion, 1)

| Year | Event        | Champion | Next_Champion |
|------|--------------|----------|---------------|
| 2004 | Discus Throw | LTU      | EST           |
| 2008 | Discus Throw | EST      | GER           |
| 2012 | Discus Throw | GER      | SWE           |
| 2004 | Triple Jump  | SWE      | POR           |
| 2008 | Triple Jump  | POR      | USA           |
| 2012 | Triple Jump  | USA      | null          |

PARTITION BY Event ありの LEAD(Champion, 1)

| Year | Event        | Champion | Next_Champion |
|------|--------------|----------|---------------|
| 2004 | Discus Throw | LTU      | EST           |
| 2008 | Discus Throw | EST      | GER           |
| 2012 | Discus Throw | GER      | null          |
| 2004 | Triple Jump  | SWE      | POR           |
| 2008 | Triple Jump  | POR      | USA           |
| 2012 | Triple Jump  | USA      | null          |

FIRST_VALUE のパーティション化

PARTITION BY Event なしの FIRST_VALUE(Champion)

| Year | Event        | Champion | First_Champion |
|------|--------------|----------|----------------|
| 2004 | Discus Throw | LTU      | LTU            |
| 2008 | Discus Throw | EST      | LTU            |
| 2012 | Discus Throw | GER      | LTU            |
| 2004 | Triple Jump  | SWE      | LTU            |
| 2008 | Triple Jump  | POR      | LTU            |
| 2012 | Triple Jump  | USA      | LTU            |

PARTITION BY Event ありの FIRST_VALUE(Champion)

| Year | Event        | Champion | First_Champion |
|------|--------------|----------|----------------|
| 2004 | Discus Throw | LTU      | LTU            |
| 2008 | Discus Throw | EST      | LTU            |
| 2012 | Discus Throw | GER      | LTU            |
| 2004 | Triple Jump  | SWE      | SWE            |
| 2008 | Triple Jump  | POR      | SWE            |
| 2012 | Triple Jump  | USA      | SWE            |

演習に進みましょう！

PostgreSQL 集計統計とウィンドウ関数