Question

以下查询“跳过”指定范围内的行。

Select Apples.*, Chiquita.color, Bananas.Time   
from Apples,  
(Select Carrot.name, Apple.type, Banana.color, MAX(Carrot.Time) as CarrotTime  
 From Apples Apple, Bananas Banana, Carrots Carrot  
where  Apple.color = Banana.color  
group by Carrot.name, Apple.type, Banana.color) Chiquita  
where rownum <= 20  
order by CarrotTime DESC;

现在，如果我将rownum修改为40，我会得到散布在日期范围之间的结果。澄清：
rownum = 20返回日期如下：

 1,12-Jun-11  
  ...  
  20, 22-May-11

rownum = 40会返回如下信息：

  1, 12-Jun-11
  ...
  20, 29-May-11  //Notice that this row 20 does not equal the information from the other row 20
  ...  
  40, 22-May-11

Answer 1

where rownum <= 20   
order by CarrotTime DESC;

Oracle在订单之前处理where子句。因此，您的查询选择它碰巧到达的前20行，然后应用CarrotTime的顺序。这通常通过将订单移动到子查询中来修复，如StevieG建议的那样。但是，当结果加入时，无法保证内部查询的顺序。

尝试另一个内部查询（未经测试）：

Select *
from (Select Apples.*, Chiquita.color, Bananas.Time
    from Apples,
    (Select Carrot.name, Apple.type, Banana.color, MAX(Carrot.Time) as CarrotTime
        From Apples Apple, Bananas Banana, Carrots Carrot
        where  Apple.color = Banana.color
        group by Carrot.name, Apple.type, Banana.color) Chiquita
    order by CarrotTime DESC)
where rownum <= 20

Answer 2

除非

，否则你的比较将不准确

a) You view the complete result set from both Queries or

b) You are sure you have only one record in the result set for a given ID..

对于您的示例，当您有两个记录时可能会有...

20, 22-May-11
20, 29-May-11

在加入后以及添加rownum＆lt; 20，你看到第一行，rownum＆lt; 40，你看到第二排。

如果您希望结果集保持一致，则可能必须在获得前20或40行之前对结果进行排序。

像...这样的东西。

select * from (
Select Apples.*, Chiquita.color, Bananas.Time   
from Apples,  
(
  Select Carrot.name, Apple.type, Banana.color, MAX(Carrot.Time) as CarrotTime  
  From Apples Apple, Bananas Banana, Carrots Carrot  
  where  Apple.color = Banana.color  
  group by Carrot.name, Apple.type, Banana.color
  order by MAX(Carrot.Time)
) Chiquita  
order by CarrotTime DESC
)
where rownum < 20 ---or rownum < 40

此外，您的查询似乎有笛卡尔联接，

1）Chiquita内部（3张桌子，只有一张加入）

2）苹果和Chiquita之间

这通常是一个错误，所以你可能想要研究一下。

Answer 3

我猜它是因为子查询中的行没有排序。试试这个：

Select Apples.*, Chiquita.color, Bananas.Time   
from Apples,  
(
  Select Carrot.name, Apple.type, Banana.color, Carrot.CarrotTime
  From Apples Apple, Bananas Banana, 
       ( 
         select name, MAX(Time) as CarrotTime 
         from Carrots 
         group by name
         order by MAX(Time), name
       ) Carrot
  where  Apple.color = Banana.color  
  group by Carrot.name, Apple.type, Banana.color
  order by Carrot.CarrotTime, Carrot.name)
) Chiquita  
where rownum <= 20  
order by CarrotTime DESC;

Oracle使用Max on date列，并按“跳过”结果排序

3 个答案: