Sql-server – SQL Server: join two tables with “valid-to” dates

join;sql server

I have two tables like so:

T1_Key, T1_Data, T1_ValidUntil
1         A       2000-01-01
1         B       2000-06-30
2         C       2005-05-31
3         D       2004-12-31
3         E       2007-04-30
3         F       2008-01-31

T2_Key, T2_Data, T2_ValidUntil
1         R       2002-03-31
2         S       2001-06-30
2         T       2003-02-28
2         U       2005-05-31
3         V       2006-09-30
3         W       2007-06-30

This defines a number of intervals where specific data is valid. For example for key 2 and date 2002-01-01, the entries (2, C, 2005-05-31) and (2, T, 2003-02-28) are valid (i.e., the entries with the next larger date). For key 2 and date 2010-01-01 no entry is valid.

For a single query like above, I just query both tables seperately and join the data in my application. However, for a maintenance application I'd like to join these two tables. The result should contains all valid intervals/combinations, like so (Key 3 is the most complex one, because there are no shared dates):

T1_Key, T1_Data, T1_ValidUntil, T2_Key, T2_Data, T2_ValidUntil
1        A        2000-01-01    1       R        2002-03-31
1        B        2000-06-30    1       R        2002-03-31
2        C        2005-05-31    2       S        2001-06-30
2        C        2005-05-31    2       T        2003-02-28
2        C        2005-05-31    2       U        2005-05-31
3        D        2004-12-31    3       V        2006-09-30  <- I1
3        E        2007-04-30    3       V        2006-09-30  <- I2
3        E        2007-04-30    3       W        2007-06-30  <- I3
3        F        2008-01-31    3       W        2007-06-30  <- I4

ASCII-Art vor Case 3 (Timelines):

     2004-12-31        2007-04-30             2008-01-31
T1  -----|-----------------|-----------------------|----------
     I1      I2       I3       I4      Nothing valid here
T2 ---------------|-----------------|--------------------------
              2006-09-30       2007-06-30

Currently I simply join using the keys, but that gives a lot of combinations which are not valid. I therefore need a way to say "Join this with the entry having the next-higher (or equal) date in the other table", and a way to do that in both directions…

The scheme is predefined and I can not change it very much. A third many-to-many table seems like redundant data for me.

Thanks in advance!

Best Answer

DECLARE @t1 TABLE (T1_key int, T1_Data char(1), T1_ValidUntil datetime)
DECLARE @t2 TABLE (T2_key int, T2_Data char(1), T2_ValidUntil datetime)

INSERT @t1 VALUES (1, 'A', '2000-01-01')
INSERT @t1 VALUES (1, 'B', '2000-06-30')
INSERT @t1 VALUES (2, 'C', '2005-05-31')
INSERT @t1 VALUES (3, 'D', '2004-12-31')
INSERT @t1 VALUES (3, 'E', '2007-04-30')
INSERT @t1 VALUES (3, 'F', '2008-01-31')

INSERT @t2 VALUES (1, 'R', '2002-03-31')
INSERT @t2 VALUES (2, 'S', '2001-06-30')
INSERT @t2 VALUES (2, 'T', '2003-02-28')
INSERT @t2 VALUES (2, 'U', '2005-05-31')
INSERT @t2 VALUES (3, 'V', '2006-09-30')
INSERT @t2 VALUES (3, 'W', '2007-06-30')

SELECT
   T1.*, T2x.*
FROM
   @t1 T1
   CROSS APPLY
   (SELECT TOP 1*
   FROM @t2 
   WHERE T1_key = T2_key AND T2_ValidUntil >= T1_ValidUntil
   ORDER BY T2_ValidUntil
   ) T2x
UNION
SELECT
   T1x.*, T2.*
FROM
   @t2 T2
   CROSS APPLY
   (SELECT TOP 1*
   FROM @t1
   WHERE T1_key = T2_key AND T1_ValidUntil >= T2_ValidUntil
   ORDER BY T1_ValidUntil
   ) T1x
Related Question