Question

我正在从SQL Movie-Rating Query Exercises Extras中进行第4季度（查找未由Chris Jackson复审的所有电影的标题。），我不知道为什么此代码不起作用：

SELECT DISTINCT movie.title
FROM movie
        INNER JOIN rating ON movie.mid = rating.mID
        INNER JOIN reviewer ON rating.rid = reviewer.rid
WHERE rating.mid NOT IN (SELECT rating.mid FROM rating WHERE rating.rid = (SELECT reviewer.rid FROM reviewer WHERE reviewer.name = 'Chris Jackson') )

输出：

title
Gone with the Wind
Snow White
Avatar

此输出不包括电影表中的电影，但不包括在评级表中的电影。因此，我怀疑这可能与JOIN子句有关。

表格：
电影

mID title                   year    director
101 Gone with the Wind      1939    Victor Fleming
102 Star Wars               1977    George Lucas
103 The Sound of Music      1965    Robert Wise
104 E.T.                    1982    Steven Spielberg
105 Titanic                 1997    James Cameron
106 Snow White              1937    <null>
107 Avatar                  2009    James Cameron
108 Raiders of the Lost Ark 1981    Steven Spielberg

审阅者

rID name
201 Sarah Martinez
202 Daniel Lewis
203 Brittany Harris
204 Mike Anderson
205 Chris Jackson
206 Elizabeth Thomas
207 James Cameron
208 Ashley White

评分

rID mID stars   ratingDate
201 101 2       2011-01-22
201 101 4       2011-01-27
202 106 4       <null>
203 103 2       2011-01-20
203 108 4       2011-01-12
203 108 2       2011-01-30
204 101 3       2011-01-09
205 103 3       2011-01-27
205 104 2       2011-01-22
205 108 4       <null>
206 107 3       2011-01-15
206 106 5       2011-01-19
207 107 5       2011-01-20
208 104 3       2011-01-02

Answer 1

首先必须使用LEFT JOIN，然后使用GROUP BY movie.mid, movie.title并将条件放在HAVING子句中：

SELECT movie.title
FROM movie
LEFT JOIN rating ON movie.mid = rating.mID
LEFT JOIN reviewer ON rating.rid = reviewer.rid
GROUP BY movie.mid, movie.title
HAVING SUM(CASE WHEN reviewer.name = 'Chris Jackson' THEN 1 ELSE 0 END) = 0

请参见demo。
结果：

> | title              |
> | :----------------- |
> | Avatar             |
> | Gone with the Wind |
> | Snow White         |
> | Star Wars          |
> | Titanic            |

Answer 2

尽管您可以使用聚合，但我建议为此使用NOT EXISTS。它接近您想要的逻辑措辞：

SELECT m.*
FROM movie m
WHERE NOT EXISTS (SELECT 1
                  FROM rating r JOIN
                       reviewer re
                       ON r.rid = re.rid
                  WHERE m.mid = r.mID AND
                        re.name = 'Chris Jackson'
                 );

从性能的角度来看，应该根本没有可比性。这应该有更好的性能。

我应该如何编写JOIN子句以使其正常工作？

2 个答案: