子查询与主查询联接

Question

I need to do a subquery, but need it's value to be based on the main query.

Basically, I am trying to get the ID , Date , and Value from a table, which we'll call Orders . However, in case there is no Value present on the specified date in there WHERE clause, then I want to know the date where the value was in (whether prior or post the specified date).

The falling query is wrong, but it might explain my logic:

SELECT 
   O.ID
  ,O.Date
  ,CASE
      WHEN O.Value IS NULL
      THEN
         (SELECT MAX(Date) 
          FROM Orders AS O1 
          WHERE O1.ID = O.ID
      ELSE O.Value END
   ) as Value
FROM Orders O
WHERE O.date = '2017-01-01'

I'm looking to get the following, for example:

ID    Value
001   100
002   300
003   12-30-2016

Answer 1

You can use CROSS APPLY/OUTER APPLY , it is enabling you to query two or more correlated queries:

select 
       o.id
     , o.date
     , case when o.value is null then t.dt else o.value end as value
from orders o outer apply ( select o1.id, max(date) dt
                            from orders as o1 
                            where o1.id = o.id 
                            group by o1.id
                          ) t
where o.date = 2017-01-01

Answer 2

You just put the closing parens in the wrong place:

SELECT 
   O.ID
  ,O.Date
  ,CASE
      WHEN O.Value IS NULL
      THEN
         (SELECT MAX(Date) 
          FROM Orders AS O1 
          WHERE O1.ID = O.ID)
      ELSE O.Value
   END as Value
FROM Orders O
WHERE O.date = '2017-01-01'

or using COALESCE:

  ,COALESCE(O.Value
           ,( SELECT MAX(Date) 
              FROM Orders AS O1 
              WHERE O1.ID = O.ID
            )
           ) as Value

Of course you still might get an error due to non-matching datatypes, then cast both O.Value and the Scalar Subquery to a VarChar.