Common Table Expressions

Manipulasi Data di SQL

Mona Khalil

Data Scientist, Greenhouse Software

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Solusi: Common Table Expressions!

Common Table Expressions

Common Table Expressions (CTE)

Tabel yang dideklarasikan sebelum kueri utama
Dinamai dan direferensikan nanti di klausa FROM

Membuat CTE

WITH cte AS (
    SELECT col1, col2
    FROM table)

SELECT
    AVG(col1) AS avg_col
FROM cte;

Ambil subkueri di FROM

SELECT
  c.name AS country,
  COUNT(s.id) AS matches
FROM country AS c
INNER JOIN (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10) AS s
ON c.id = s.country_id
GROUP BY country;

| country     | matches |
|-------------|---------|
| England     | 3       |
| Germany     | 1       |
| Netherlands | 1       |
| Spain       | 4       |

Letakkan di awal

(
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10
)

Letakkan di awal

WITH s AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10
)

Tampilkan CTE-nya

WITH s AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10
)
SELECT
  c.name AS country,
  COUNT(s.id) AS matches
FROM country AS c
INNER JOIN s
ON c.id = s.country_id
GROUP BY country;

| country     | matches |
|-------------|---------|
| England     | 3       |
| Germany     | 1       |
| Netherlands | 1       |
| Spain       | 4       |

Tampilkan semua CTE

WITH s1 AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10),
s2 AS (                              -- Subkueri baru
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) <= 1
)
SELECT
  c.name AS country,
  COUNT(s1.id) AS high_scores,
  COUNT(s2.id) AS low_scores         -- Kolom baru
FROM country AS c
INNER JOIN s1
ON c.id = s1.country_id
INNER JOIN s2                        -- Join baru
ON c.id = s2.country_id
GROUP BY country;

Mengapa memakai CTE?

Dieksekusi sekali
- Lalu disimpan di memori
- Meningkatkan performa kueri
Memperbaiki organisasi kueri
Dapat mereferensikan CTE lain
Dapat mereferensikan dirinya sendiri (SELF JOIN)

Ayo berlatih!

Manipulasi Data di SQL