Question

In principle I have 2 tables T1 and T2, each containing a date-field, lets call it date:

T1: date | somekey  | data ... 
T2: date | somekey  | data ...

I want to (left)join T1 and T2, such that the result has all rows from T1.

Now for each row from T1 I need the values from the (exactly one) row of T2 joined where ABS(DATEDIFF(T1.date,T2.date)) is minimal and T1.somekey=T2.somekey (or null in case there is no such row in T2)

Answer 1

Don’t do it in the JOIN clause, just put it in the WHERE clause.

SELECT
*
FROM
T1
LEFT JOIN T2 ON T1.somekey = T2.somekey
WHERE
ABS(DATEDIFF(T1.date,T2.date)) = 1 /*or whatever "minimal" means for you*/
/*or like this:*/                < $yourMinimalValue

EDIT:

I think I get what you want to do, but subqueries are the way to go here:

SELECT
*
FROM
T1
LEFT JOIN T2 ON T1.somekey = T2.somekey
WHERE
ABS(DATEDIFF(T1.date,T2.date)) = (SELECT MIN(DATEDIFF(TT1.date, TT2.date)) FROM  T1 AS TT1 LEFT JOIN T2 AS TT2 ON TT1.somekey = TT2.somekey)