Question

I have two tables, a header and a detail table:

header
id
name

detail
headerID
dueDate
extensionDate

I’m trying to make a query that will pull the ID and Name of a header who’s detail dueDate is 5 days from now (if extensionDate is null), or if there is an extensionDate, pull the header who has an extensionDate that is 5 days from now. Can this be done in a single query? I hope this makes sense.
I’m having trouble with determining if there is an extensionDate, and checking that, or if not, check the original dueDate. Thank you for your help!

Answer 1

select id, name
from header
where exists (
    select *
    from detail
    where headerid=header.id
    and (extensiondate = DATEDIFF(d, -5, getdate())
     or (duedate = DATEDIFF(d, -5, getdate()) and extensiondate is null)))

Notes:

1. DATEDIFF(d, -5, getdate() gives you a date exactly 5 days in the future, without the time portion.
2. If you had indexes on extensiondate and duedate, this can utilise them separately. Compare this with the other queries presented here which will perform a table scan.

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EDIT (also contains alternative query)

Why EXISTS? First create these test tables. About 8% of details will match the conditions, which represents 5% of header records (unique).

create table header (id int, name varchar(max))
insert header select distinct number, replicate(number, 10) from master..spt_values
create table detail (id int identity primary key, headerid int, duedate datetime, extensiondate datetime)
create index ix_duedate on detail(duedate)
create index ix_extensiondate on detail(extensiondate)
insert detail (headerid, duedate, extensiondate)
select number,
    DATEDIFF(d,-convert(bigint,convert(varbinary(max),NEWID()))%20 - 10, GETDATE()),
    case when -convert(bigint,convert(varbinary(max),NEWID()))%2 = 1 then null else
    DATEDIFF(d,-convert(bigint,convert(varbinary(max),NEWID()))%20 - 10, GETDATE()) end
from master..spt_values

Check the execution plans for the following two, using Ctrl-M

select id, name
from header
where exists (
    select *
    from detail
    where headerid=header.id
    and (extensiondate = DATEDIFF(d, -5, getdate())
     or (duedate = DATEDIFF(d, -5, getdate()) and extensiondate is null)))

select distinct h.id, h.name
from header h
join detail d on d.headerid = h.id
  and (d.extensiondate = DATEDIFF(d, -5, getdate())
     or (d.duedate = DATEDIFF(d, -5, getdate()) and d.extensiondate is null))

The first one is just slightly faster