共通テーブル式（CTE）

SQL のデータ操作

Mona Khalil

Data Scientist, Greenhouse Software

サブクエリを追加する場合...

クエリはすぐに複雑に
- 情報の把握も困難

解決策： 共通テーブル式！

共通テーブル式（CTE）

共通テーブル式（CTE）

メインクエリの前に宣言するテーブル
名前を付け、あとで FROM 句から参照

CTE の設定

WITH cte AS (
    SELECT col1, col2
    FROM table)

SELECT
    AVG(col1) AS avg_col
FROM cte;

FROM でサブクエリを取得

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       |

先頭に配置

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

先頭に配置

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

CTE を表示

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       |

すべての CTE を表示

WITH s1 AS (
  SELECT country_id, id 
  FROM match
  WHERE (home_goal + away_goal) >= 10),
s2 AS (                              -- New subquery
  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         -- New column
FROM country AS c
INNER JOIN s1
ON c.id = s1.country_id
INNER JOIN s2                        -- New join
ON c.id = s2.country_id
GROUP BY country;

CTE を使う理由

1 回だけ実行
- CTE はその後メモリに保存
- クエリのパフォーマンスを向上
クエリの整理を改善
他の CTE を参照
自身も参照可能（SELF JOIN）

練習しましょう！

SQL のデータ操作