Menerapkan agregat statistik pada data deret waktu

Analisis Deret Waktu di PostgreSQL

Jasmin Ludolf

Content Developer, DataCamp

Agregat statistik

Fungsi agregat: MIN(), MAX(), SUM(), AVG(), COUNT()
Agregat statistik: mean dan median

Ilustrasi menampilkan empat orang di sekitar papan dengan visual analitik dan statistik.

Menghitung rata-rata

SELECT
  AVG(views)::INTEGER as avg_views
  title
FROM dc_news_fact
JOIN dc_news_dim USING(id)
GROUP BY title
ORDER BY avg_views DESC;

|avg_views|title                      |
|---------|---------------------------|
|      207|For the Wealthiest, a Pr...|
|       12|Pet surrenders on rise a...|
|       12|Argentina's New Presiden...|
|        7|These 5 charts prove tha...|
|        7|How Is the Economy Doing...|

Rata-rata jumlah tayangan per hari

WITH day_views AS (
  SELECT id, ts::DATE AS date, SUM(views) AS views
  FROM dc_news_fact
  GROUP BY id, date
)
SELECT
  AVG(views)::INTEGER AS avg
  title
FROM day_views JOIN dc_news_dim USING(id)
GROUP BY title
ORDER BY avg DESC;

Rata-rata jumlah tayangan per hari

| avg|title                                                            |
|----|-----------------------------------------------------------------|
|9855|For the Wealthiest, a Private Tax System That Saves Them Billions|
| 579|Pet surrenders on rise as Fort McMurray's economy falls          |
| 574|Argentina's New President Moves Swiftly to Shake Up the Economy  |
| 352|How Is the Economy Doing? Politics May Decide Your Answer        |
| 348|These 5 charts prove that the economy does better under ...      |

Rata-rata jumlah tayangan per hari

SELECT
  (SUM(views)/COUNT(DISTINCT ts::DATE))::INTEGER as avg,
  title
FROM dc_news_fact JOIN dc_news_dim USING(id)
GROUP BY title 
ORDER BY avg DESC;

|avg |title                                                            |
|----|-----------------------------------------------------------------|
|9855|For the Wealthiest, a Private Tax System That Saves Them Billions|
| 579|Pet surrenders on rise as Fort McMurray's economy falls          |
| 574|Argentina's New President Moves Swiftly to Shake Up the Economy  |
...

Median diskret dan kontinu

Median diskret: nilai pertama yang paling dekat ke nilai tengah
Median kontinu: nilai yang membagi dataset menjadi dua sama besar

Jumlah elemen ganjil

Deret = (1, 2, 3, 4, 5)
Median diskret = 3
Median kontinu = 3

Jumlah elemen genap

Deret = (1, 2, 3, 4)
Median diskret = 2
Median kontinu = 2,5

Fungsi agregat ordered-set

PERCENTILE_DISC()
PERCENTILE_CONT()
Fungsi agregat ordered-set: PERCENTILE_DISC(fraction) WITHIN GROUP (ORDER BY field)

SELECT
  PERCENTILE_CONT(0.5) WITHIN GROUP
  (ORDER BY value) AS median_cont,
  PERCENTILE_DISC(0.5) WITHIN GROUP
  (ORDER BY value) AS median_disc
FROM
(
  VALUES
  (1,1), (1,2), (1,3), (1,4), (1,5),
  (2,1), (2,5), (2,7), (2,11), (2,11)
) AS t (id, value)
GROUP BY id;

Fungsi agregat ordered-set

|median_cont|median_disc|
|-----------|-----------|
|        3.0|          3|
|        7.0|          7|

Median, kuantil, persentil, kuartil

Median adalah jenis persentil
Persentil adalah jenis kuantil

Kuantil: membagi sampel menjadi subset hampir sama besar
- kuartil (empat subset)
- desil (sepuluh subset)

Menghitung kuartil

SELECT
  PERCENTILE_DISC(0.25) WITHIN GROUP (ORDER BY value) AS ptile_25,
  PERCENTILE_DISC(0.5) WITHIN GROUP (ORDER BY value) AS ptile_50,
  PERCENTILE_DISC(0.75) WITHIN GROUP (ORDER BY value) AS ptile_75
FROM
(
  VALUES 
  (1,1), (1,2), (1,3), (1,4), (1,5)
) AS t (id, value)
GROUP BY id;

|ptile_25|ptile_50|ptile_75|
|--------|--------|--------|
|       2|       3|       4|

Menghitung array kuartil diskret

SELECT
  PERCENTILE_DISC(ARRAY[0.25, 0.5, 0.75]) 
  WITHIN GROUP (ORDER BY value) AS median_disc
FROM (
  VALUES
  (1,1), (1,2), (1,3), (1,4), (1,5) )
  AS t (id, value)
GROUP BY id;

|median_disc|
|-----------|
|{2,3,4}    |

Ayo berlatih!

Analisis Deret Waktu di PostgreSQL