Question

虽然我想在我的问题上找到一个好的答案Analog of OUTER APPLY in other RDBMS (not SQL Server)我找到了相当不错的PostgreSQL解决方案：

create table Transactions
(
    ID int, Date timestamp, Amount decimal(29, 2), Amount2 decimal(29, 2)
);

insert into Transactions (ID, Date, Amount, Amount2)
select 1, current_timestamp, 100.00, null union all
select 2, current_timestamp, 25.00, 75.00;

select
    T.ID,
    T.Date,
    unnest(array[T.Amount, T.Amount2]) as Amount
from Transactions as T

SQL FIDDLE

重点是将一些列转换为具有最可读和优雅代码的行。但我不希望将空列视为行。有没有什么办法可以在查询的WHERE子句中使用来自unfst的值？

Answer 1

您可以使用子查询和where过滤掉NULL值：

select id, date, Amount
from (select t.*, unnest(array[T.Amount, T.Amount2]) as Amount
      from Transactions as T
     ) t
where Amount is not null;

Postgres不允许unnest子句中的where方向。

编辑：

Unnest使用数组的长度来确定行数。您可以使用标准SQL而不是子查询来执行此操作，但您可能会发现它更加混乱：

      select T.ID, T.Date,
             (case when n = 1 then T.Amount
                   when n = 2 then T.Amount2
              end) as Amount
      from Transactions T cross join
           (select 1 as n union all select 2) n
      where (case when n = 1 then T.Amount
                  when n = 2 then T.Amount2
             end) is not null;

在WHERE中引用unexst（或其他srf）的值

1 个答案: