Question

我有一张名为A10的桌子。我想创建一个名为A10T1的视图，它将显示没有cat值为Y的人的每个状态的状态缩写。我需要使用子查询但不会输出。

我目前的代码是：

CREATE VIEW A10T1 AS
SELECT STATE
FROM A10
WHERE STATE IS NOT NULL AND ID IN
 (SELECT ID
  FROM A10
  GROUP BY ID
  HAVING COUNT(ID) =
   (SELECT MIN(CATLOVERS)
    FROM (SELECT COUNT(ID) AS CATLOVERS
          FROM A10
          WHERE CAT = 'Y'
          GROUP BY STATE)))
ORDER BY STATE;

Answer 1

可惜有人刚刚给你一份作业答案，你可以通过这种方式练习复制/粘贴，但学习不多。

为什么您的查询失败？

自行运行此查询：

SELECT COUNT(ID) AS CATLOVERS
      FROM A10
      WHERE CATLOVERS = 'Y'
      GROUP BY STATE

从子查询得到的是一列正整数，除非没有状态有catlovers ='Y'，如果发生这种情况，子查询将返回NULL。

现在考虑

SELECT MIN(CATLOVERS)
FROM (SELECT COUNT(ID) AS CATLOVERS
      FROM A10
      WHERE CATLOVERS = 'Y'
      GROUP BY STATE)x

因此，一系列正整数的MIN（）可能是1，并且此子查询将返回单行1，除非第一个子查询返回NULL，这也会导致NULL。

因此，如果较早的子查询为NULL，则下一个子查询将仅列出其中count（ID）=最小或没有行的ID

SELECT ID
  FROM A10
  GROUP BY ID
  HAVING COUNT(ID) =
   (SELECT MIN(CATLOVERS)
    FROM (SELECT COUNT(ID) AS CATLOVERS
          FROM A10
          WHERE CATLOVERS = 'Y'
          GROUP BY STATE)x)

尝试独立运行子查询以研究发生的情况。

请参阅：http://data.stackexchange.com/stackoverflow/query/577188

Answer 2

首先你需要创建视图：

CREATE OR replace VIEW a10t1 
AS 
  SELECT state 
  FROM   a10 
  WHERE  state IS NOT NULL 
         AND id IN (SELECT id 
                    FROM   a10 
                    GROUP  BY id 
                    HAVING Count(id) = (SELECT Min(catlovers) 
                                        FROM   (SELECT Count(id) AS CATLOVERS 
                                                FROM   a10 
                                                WHERE  cat = 'Y' 
                                                GROUP  BY state))) 
  ORDER  BY state;

然后你可以从这个视图中选择：

select * from a10t1

SQL子查询：没有Cats的状态

2 个答案: