使用 EXISTS 的查詢

以資料驅動的 SQL 決策制定

Irene Ortner

Data Scientist at Applied Statistics

EXISTS

相關子查詢的一種特殊情況。
用來檢查相關子查詢的結果是否為空。
回傳：TRUE 或 FALSE
TRUE = 非空 -> 會選出外層查詢的該列。
FALSE = 空
不考慮 SELECT 子句中的欄位—請用 SELECT *

至少有一個評分的電影

SELECT *
FROM movies AS m
WHERE EXISTS
    (SELECT *
     FROM renting AS r
     WHERE rating IS NOT NULL 
     AND r.movie_id = m.movie_id);

至少有一個評分的電影

SELECT *
FROM renting AS r
WHERE rating IS NOT NULL 
AND r.movie_id = 11;

| renting_id | customer_id | movie_id | rating | renting_price |
|------- ----|-------------|----------|--------|---------------|

至少有一個評分的電影

SELECT *
FROM renting AS r
WHERE rating IS NOT NULL 
AND r.movie_id = 1;

| renting_id | customer_id | movie_id | rating | renting_price |
|------------|-------------|----------|--------|---------------|
| 71         | 111         | 1        | 5      | 2018-07-21    |
| 170        | 36          | 1        | 10     | 2018-10-18    |

EXISTS 查詢（有結果）

SELECT *
FROM movies AS m
WHERE EXISTS
    (SELECT *
     FROM renting AS r
     WHERE rating IS NOT NULL 
     AND 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          |

NOT EXISTS

TRUE = 資料表為空 -> 會選出外層查詢的該列。

SELECT *
FROM movies AS m
WHERE NOT EXISTS
    (SELECT *
     FROM renting AS r
     WHERE rating IS NOT NULL 
     AND r.movie_id = m.movie_id);

| movie_id | title    | genre  | runtime | year_of_release | renting_price |
|----------|----------|--------|---------|-----------------|---------------|
| 11       | Showtime | Comedy | 95      | 2002            | 1.79          |

一起來練習吧！

以資料驅動的 SQL 決策制定