Question

问题是sqlzoo，我写了下面的代码，但我觉得它太冗了

SELECT year, freq 
FROM (SELECT yr AS year,count(title) AS freq
FROM movie, actor, casting
WHERE name= 'John Travolta'
AND movie.id=movieid
AND actor.id=actorid
GROUP BY yr) AS a
WHERE freq=(
SELECT MAX(freq)
FROM (SELECT yr AS year,count(title) AS freq
FROM movie, actor, casting
WHERE name= 'John Travolta'
AND movie.id=movieid
AND actor.id=actorid
GROUP BY yr) AS b
)

为什么不能这样？

SELECT year, freq 
FROM (SELECT yr AS year,count(title) AS freq
FROM movie, actor, casting
WHERE name= 'John Travolta'
AND movie.id=movieid
AND actor.id=actorid
GROUP BY yr) AS a
WHERE freq=(
SELECT MAX(freq)
FROM a
)

Answer 1

在这种情况下，使用CTE（公用表表达式）可能会有所帮助。这是您可以重用子查询的唯一方法。了解如何使用ROW_NUMBER查找最大频率。我还将旧学校FROM A, B, C WHERE ...更新到新学校FROM A INNER JOIN B ...（我不是100％确定JOIN标准是正确的。）

WITH a AS
(
    SELECT 
        yr AS year, 
        COUNT(title) AS freq
    FROM 
        movie
    INNER JOIN 
        casting ON movie.id = casting.movieid
    INNER JOIN 
        actor ON actor.id = casting.actorid
    WHERE 
        name = 'John Travolta'
    GROUP BY 
        yr),
b AS
(
     SELECT 
         year, freq,
         ROW_NUMBER() OVER (ORDER BY freq DESC) as RowNum
     FROM a
)
SELECT year, freq 
FROM  b
WHERE RowNum = 1

Answer 2

无论何时编写子查询，首先评估内部查询，然后评估外部查询。在第二个查询中，您使用的是实际上不存在的别名“a”。这就是您将收到错误的原因第二个查询，你不能使用它。第一个查询在语法上是正确的。

sqlzoo joinII exercise - 电影数据库4a

2 个答案: