从SQL表中提取当前数据值和相应的子值

Question

If this has been answered, just link me to the answer and I'll sit there going 'Why didn't I think of that for the search?'

I have two databases which I'm trying to extract data from using SQL. The data bases are a parent and child.

Per - Parent, Hed - Child

Per has the following columns of information: Site, varfrom, varto, sdate, table and others which aren't important

Hed has the following columns of information: Site, varfrom, varto, table, release, name, and others which aren't important

Per example - the true and false are one of the extra columns

STATION VARFROM VARTO   SDATE   STIME   REFSTN  REFTAB
221209A 100.00  141 8/11/2006   1200.00 221209A 23  FALSE
221209A 100.00  141 22/04/2007  400.00  221209A 23  TRUE
221209A 100.00  141 30/04/2007  1600.00 221209A 24  FALSE
221209A 100.00  141 2/11/2007   0.00    221209A 24  TRUE
221209A 100.00  141 5/11/2007   2000.00 221209A 25  FALSE
221209A 100.00  141 24/11/2008  0.00    221209A 26  FALSE
221209A 100.00  141 16/02/2010  45.00   221209A 27  FALSE
221210A 100.00  141 22/08/1972  1545.00 221210A 1   FALSE
221210A 100.00  141 3/06/1978   1500.00 221210A 2   FALSE
221210A 100.00  141 5/07/1982   1130.00 221210A 1   FALSE
221210A 100.00  141 28/07/1984  1225.00 221210A 3   FALSE
221210A 100.00  141 24/02/1987  1200.00 221210A 2   FALSE
221210A 100.00  141 28/04/1988  1600.00 221210A 4   FALSE
221210A 100.00  141 5/01/1992   1430.00 221210A 3   FALSE

Hed example

STATION     VARFROM VARTO   TABLE   RELEASE NAME
221209A     100.00  141     23      0       23.0
221209A     100.00  141     24      0       24.0
221209A     100.00  141     25      2       25.02
221209A     100.00  141     25      1       25.01
221209A     100.00  141     25      0       25.00
221209A     100.00  141     26      0       26.00
221209A     100.00  141     27      0       27.00
221210A     100.00  141     1       3        1.03
221210A     100.00  141     1       2        1 B
221210A     100.00  141     2       6        2.06
221210A     100.00  141     2       5        2.05
221210A     100.00  141     2       4        2 D
221210A     100.00  141     3       1        3.01
221210A     100.00  141     3       0        3
221210A     100.00  141     4       2        4.02
221210A     100.00  141     4       1        4 A

So what I want is for it to return the most recent sdate for each site, then give me the current name for each site

So the answer for the example would be

Site    Varfrom Varto sdate      name
221209A 100.00  141   16/02/2010 27.00
221210A 100.00  141   5/01/1992   3.01

It doesn't matter if site comes from Per or Hed.

Currently I have any number of coding attempts which give me out everything, except that the name is incorrect. Below is one example. I just can't see how to make it give me the latest sdate with the right name.

select
 hed.site,
 hed.name,
 max(per.sdate) as "Period",
 hed.varfrom,
 hed.varto
from hed inner join per 
on hed.site=per.site AND
 hed.table=per.reftab
where
 hed.varfrom=100.00 
 AND  hed.varto=141

Any help would be very much appreciated.

Answer 1

Try this

;with cte as (
select site,Varfrom ,Varto,reftable,sdate,r=ROW_NUMBER() over(partition by site order by sdate desc) from per
)

select per.site,per.Varfrom ,per.Varto,hed.name from cte per
inner join hed on per.site = hed.site and per.reftable=hed.reftable and per.r=1