sql - 结合 select 语句得到的结果集创建视图

Question

对于 4 个不同的表，我有 4 个 Select 语句，每个 Select 查询给出满足指定条件的最新记录，例如：

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

现在我必须组合所有 4 个查询的结果集并从组合结果集中创建一个视图。

Answer 1

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 ....)
)

Answer 2

一些数据库不允许您在联合查询中提供“order by”子句。

如果您按 col1 desc 排序，则它可能是您可以应用 min() 或 max() 的某种类型的列。

如果是这种情况，下面可以解决您的问题（如果记录不多，或者如果表很大，则“col1”和“some_column”被索引。）

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')
)