Question

DB: PostgreSQL

Suppose I have

table A(integer a_id, timestamp x, timestamp y) 
table B(int b_id, timestamp n, timestamp m)

m can be null

I want to fetch the data with this structure

SELECT a_id, j.b_id, k.b_id 
FROM A, B AS j, B AS k 
WHERE (x BETWEEN j.n AND j.m) AND (y BETWEEN k.n AND k.m.)

When (x BETWEEN j.n AND j.m) AND (y BETWEEN k.n AND k.m.)‘s matches aren’t not found

I still want the query to fetch the data with null j.b_id or null k.b_id

How do I do that?

Answer 1

My interpretation of the cryptic question …

SELECT a_id, j.b_id, k.b_id 
FROM   a
LEFT   JOIN b j ON a.x BETWEEN j.n AND j.m
LEFT   JOIN b k ON a.y BETWEEN k.n AND k.m

If no matching row is found in j or k, NULL is delivered for j.b_id or k.b_id respectively.

The query suffers from the principal design flaw that multiple matches in b will multiply the number of rows returned in a CROSS JOIN fashion.