相結合的結果集創建一個視圖

Question

我有4個4頁不同的表 Select語句中的每個SELECT查詢給定條件 爲前最新記錄會議：

Select TOP 1 * from table where column_name = 'something' order by col1 DESC;

現在我必須結合所有4個查詢的結果集，並從組合結果集中創建一個視圖。

Answer 1

有些數據庫不允許您在聯合查詢中提供「order by」子句。

如果您按col1 desc進行排序，則可能是某種類型的列可以應用min（）或max（）。

如果是這樣的話，下面可以解決您的問題（如果沒有太多記錄，或者，如果表是巨大的，所有「col1」和「some_column」被編入索引。）

create view some_view as 
(
select * from table1 
    where some_column = 'something' 
    and col1 = (select max(col1) from table1 where some_column = 'something') 
UNION ALL 
select * from table2 
    where some_column = 'something' 
    and col1 = (select max(col1) from table2 where some_column = 'something') 
UNION ALL 
select * from table3 
    where some_column = 'something' 
    and col1 = (select max(col1) from table3 where some_column = 'something') 
UNION ALL 
select * from table4 
    where some_column = 'something' 
    and col1 = (select max(col1) from table4 where some_column = 'something') 
) 

Answer 2

create view some_view as 
(
(select TOP 1 * from table1 where ....) 
UNION ALL 
(select TOP 1 * from table2 where ....) 
UNION ALL 
(select TOP 1 * from table3 where ....) 
UNION ALL 
(select TOP 1 * from table4 where ....) 
)