Question

我有4个不同表的4个Select语句每个Select查询给出最新记录满足指定条件例如：

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

某些DB不允许您在联合查询中提供“order by”子句。

如果您按col1 desc订购，可能是某种类型的列可以应用min（）或max（）。

如果是这种情况，下面可以解决您的问题（如果没有太多记录，或者，如果表格很大，“col1”和“some_column”将被编入索引。）

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')
)

组合select语句获得的结果集来创建视图

2 个答案: