Question

Following Table (let’s name it validity_period):

-------------------------------
id | valid_from  | valid_until
-------------------------------
1    2012-11-12    2012-12-02
2    2012-12-03    NULL
3    2012-12-15    2012-12-21

(valid_from is not nullable; valid_until is nullable, but don’t have to be null)

Now I want to find out which entry is valid today (2012-12-19). From the logical sight of view it has to be entry 3, because the entries can overlap each other but only one entry is valid on one day. (On 2012-12-22 it has to be entry 2 which is valid.)
Note that all entries can have a valid_until, but there can’t be more than one entry where valid_until is NULL.

How would I perform this in a SQL-Query? (If possible in SQLAlchemy, but I also can translate it myself from raw SQL)

(I’m using PostgreSQL 9.1)

EDIT: Here my final resolution. Thanks to all contributors!

SELECT * 
    FROM validity_period 
    WHERE valid_from <= CURRENT_DATE AND 
          valid_until >= CURRENT_DATE
UNION
SELECT * 
    FROM validity_period 
    WHERE valid_from <= CURRENT_DATE AND 
          valid_until IS NULL
ORDER BY valid_from DESC 
LIMIT 1;

Answer 1

Try something like this:

SELECT  *
FROM    validity_period
WHERE   valid_from <= current_date
        AND valid_to >=current_date    
UNION 

--Gets the last instance where valid_until is null
SELECT * 
FROM validity_period 
WHERE   valid_from <= current_date
        AND valid_to IS NULL
ORDER BY id DESC LIMIT 1 

Simply remove the DESC if you want the first rather than the last null field