如何将子查询的条件移出子查询？

Question

在 Oracle 中，我有一个相当大的 select 语句。我想将它保存为一个视图，这样我就可以从 .NET 程序中调用它，并将它用作我们必须生成的一些统计数据的基础。使用视图时，它总是按日期过滤。

我想要这样的东西：

select * from my_view
where my_view.date = '2014-01-01';

问题是我需要在子查询的where条件中使用日期作为参数：

select * from table1
left outer join (
    select * from table2
    where table2.date = :somedate)
  on table1.Id = table2.Id;

...不能做成视图。

有没有办法，我可以将日期比较移到子查询之外，而不会弄乱左外连接？

不该做什么：

这会创建一个内部连接 ​​- 这不是我想要的：

select * from table1
left outer join 
  table2
  on table1.Id = table2.Id
where table2.date = '2014-01-01';

此选择过滤掉 table1 中的行，这些行不在指定日期的行上连接，但在另一个日期的行上连接：

select * from table1
left outer join 
  table2
  on table1.Id = table2.Id
where table2.Id is null
   or table2.date = '2014-01-01';

Answer 1

有多种方法可以对视图进行参数化 - 例如使用包或应用程序上下文（您可以使用内置上下文 CLIENTCONTEXT）。但是所有这些方法都需要完全控制参数，并且您必须使用附加功能设置参数值，这可能是一个严重的缺点：

SQL> create or replace view param_view
  2  as
  3  select d.name, e.firstname, e.lastname, e.email
  4  from department d left join employee e
  5  on (d.id = e.departmentid
  6  and e.firstname = sys_context('CLIENTCONTEXT','EMPNAME'))
  7  /

View created.

SQL> exec dbms_session.set_context('CLIENTCONTEXT','EMPNAME','Scott')

PL/SQL procedure completed

SQL> select * from param_view;

NAME                 FIRSTNAME  LASTNAME             EMAIL                      
-------------------- ---------- -------------------- --------------------       
Department A         Scott      Tiger                xxx@gmail.com              
Department B                                                                    
Department C                                                                    

SQL> exec dbms_session.set_context('CLIENTCONTEXT','EMPNAME','Allen')

PL/SQL procedure completed

SQL> select * from param_view;

NAME                 FIRSTNAME  LASTNAME             EMAIL                      
-------------------- ---------- -------------------- --------------------       
Department A         Allen      Dirk                 yyy@gmail.com              
Department B                                                                    
Department C   

或

SQL> create or replace package pck_test
  2  is
  3   function get return varchar2;
  4   procedure set (x in varchar2);
  5  end;
  6  /

Package created.

SQL> create or replace package body pck_test
  2  is
  3   name employee.firstname%type;
  4  
  5   function get return varchar2
  6   is
  7   begin
  8     return name;
  9   end;
 10  
 11   procedure set (x in varchar2)
 12   is
 13   begin
 14     name := x;
 15   end;
 16  
 17  end;
 18  /

Package body created.

SQL> create or replace view param_view
  2  as
  3  select d.name, e.firstname, e.lastname, e.email
  4  from department d left join employee e
  5  on (d.id = e.departmentid
  6  and e.firstname = pck_test.get)
  7  /

View created

SQL> exec pck_test.set('Scott')

PL/SQL procedure completed.

SQL> select * from param_view;

NAME                 FIRSTNAME  LASTNAME             EMAIL                      
-------------------- ---------- -------------------- --------------------       
Department A         Scott      Tiger                xxx@gmail.com              
Department B                                                                    
Department C                                                                    

SQL> exec pck_test.set('Allen')

PL/SQL procedure completed.

SQL> select * from param_view;

NAME                 FIRSTNAME  LASTNAME             EMAIL                      
-------------------- ---------- -------------------- --------------------       
Department A         Allen      Dirk                 yyy@gmail.com              
Department B                                                                    
Department C                                                                    

SQL> select d.name, e.firstname, e.lastname, e.email
  2  from department d left join employee e
  3  on (d.id = e.departmentid)
  4  /

NAME                 FIRSTNAME  LASTNAME             EMAIL                      
-------------------- ---------- -------------------- --------------------       
Department A         Allen      Dirk                 yyy@gmail.com              
Department A         Scott      Tiger                xxx@gmail.com              
Department B                                                                    
Department C  

Answer 2

如果您只是想避免在底部创建内连接，请添加合并语句，例如：

coalesce(table1.date,'2015-01-28') = '2015-01-28'

这将不会创建内部连接，因为所有空值仍将返回。

Answer 3

已删除原始回复。 这是我想出的

create view my_view as
select * from (
select t1.*, t2.*, to_date(null) filter_date from table1 t1 left outer join table2 t2
on t2.id <> t2.id
union all
select t1.*, t2.*, t2.date filter_date from table1 t1 inner outer join table2 t2
on t1.id = t2.id)

1. 第一个数据集是从 t1 获取所有记录

2. 第二个数据集是从 t1 与 t2 内连接获取所有记录 因此，在查询中，您将始终有 1 或 2 条记录可供选择。如果有 2 条记录，请选择带日期的一条：

   select * from (
   select *, row_number() over (partition by id order by filter_date NULLS LAST) rnum
   from my_view where filter_date = :date or filter_date is null
   ) where rnum = 1
   

或

   select * from (
    select *, rownum rnum
    from my_view where filter_date = :date or filter_date is null
    order by filter_date NULLS LAST
    ) where rnum = 1