如果存在这样的行，则仅获得满足条件的行；如果不存在，则仅获得满足另一条件的行

Question

这听起来像是一个简单的问题，但我只是找不到正确的方法。 给出简化表

with t as  (
select   ordernumber, orderdate,  case when ordertype in (5,21) then 1 else 0 end is_restore , ordertype, row_number() over(order by orderdate) rn from
( 
select to_date('29.08.08','DD.MM.YY') orderdate,'313' ordernumber, 1  as ordertype  from dual union all
select to_date('13.03.15','DD.MM.YY') orderdate, '90/4/2' ordernumber, 5  as ordertype from dual
)
)
select * from t -- where clause  should be here 

每行is_restore保证为1或0。 如果表中有is_restore = 1的行，则选择订单号，该行的订单日期，然后再选择其他任何内容。 如果表中没有is_restore = 1的行，则选择ordernumber，rn = 1的行的orderdate（保证在表中存在rn = 1的行）  鉴于上述要求，我需要在where子句中获取以下内容？

Answer 1

您可以使用ROW_NUMBER：

CREATE TABLE t
AS
select ordernumber, orderdate,
    case when ordertype in (5,21) then 1 else 0 end is_restore, ordertype,
    row_number() over(order by orderdate) rn
  from ( 
     select to_date('29.08.08','DD.MM.YY') orderdate,'313' ordernumber,
           1  as ordertype 
     from dual union all
     select to_date('13.03.15','DD.MM.YY') orderdate, '90/4/2' ordernumber,
            5  as ordertype
     from dual);

-------------------

with cte as (
  select t.*,
    ROW_NUMBER() OVER(/*PARTITION BY ...*/ ORDER BY is_restore DESC, rn) AS rnk
  from t
)
SELECT *
FROM cte
WHERE rnk = 1;

db<>fiddle demo

Answer 2

这里是sql，它不使用窗口函数，对于那些数据库不支持OVER（...）或有索引字段（基于查询的数据库）的人来说，它可能会有用。

SELECT
* 
FROM t
WHERE t.is_restore = 1 
OR (
 NOT EXISTS (SELECT 1 FROM t WHERE t.is_restore = 1)
 AND t.rn = 1
)