相依巢狀查詢

以資料驅動的 SQL 決策制定

Bart Baesens

Professor Data Science and Analytics

相依查詢

放在內層查詢 WHERE 子句的條件。
會參照外層查詢中某個資料表的欄位。

相依查詢範例

租借次數大於 5 的電影數量

SELECT *
FROM movies as m
WHERE 5 < 
  (SELECT COUNT(*)
  FROM renting as r
  WHERE r.movie_id=m.movie_id);

評估內層查詢

SELECT COUNT(*)
FROM renting as r
WHERE r.movie_id = 1;

| count |
|-------|
| 8     |

評估外層查詢

租借次數大於 5 的電影

SELECT *
FROM movies as m
WHERE 5 < 
    (SELECT COUNT(*)
    FROM renting as r
    WHERE r.movie_id = m.movie_id);

| movie_id | title                 | genre  | runtime | year_of_release | renting_price |
|----------|-----------------------|--------|---------|-----------------|---------------|
| 1        | One Night at McCool's | Comedy | 93      | 2001            | 2.09          |
| 2        | Swordfish             | Drama  | 99      | 2001            | 2.19          |

少於 5 次的租借

選出租借次數少於 5 次的電影。

SELECT *
FROM movies as m
WHERE 5 > 
    (SELECT COUNT(*)
    FROM renting as r
    WHERE r.movie_id = m.movie_id);

| movie_id | title           | genre              | runtime | year_of_release | renting_price |
|----------|-----------------|--------------------|---------|-----------------|---------------|
| 17       | The Human Stain | Mystery & Suspense | 106     | 2003            | 1.99          |
| 20       | Love Actually   | Comedy             | 135     | 2003            | 2.29          |

一起來練習吧！

以資料驅動的 SQL 決策制定