你如何连接表，以便右表值取决于左表中的两个不同行

Question

Sorry for messy title, if you are a moderator and know a better title, feel free to change.

Say, we have two SQL tables

intervals           vals       
--------            -------    
since               val        
--------            -------    
 1                  1          
 4                  2          
 8                  3
 20                 4          
 ...                ...
 500                100

I want to make a join so that "since" field from intervals table would be a lower bound for a "val". And "since" values that have no "val" that is larger would not appear. See what I want to get:

since val
--------------
1     1
1     2
1     3
4     4
4     5
4     6
4     7
8     8
8     9
.....

How do I do it in generic SQL? Postgres-only solution will fit as well.

Answer 1

Rather than think of it as "multiple rows", think of it as a range .

This does what you want:

select i.since, v.val
from intervals i
join vals v on v.val between i.since and 
    (select min(since) - 1 from intervals where since > i.since)
order by 1, 2;

Test code (run on postgres as per OP's question):

create table intervals (since int);
create table vals (val int);
insert into intervals values (1), (4), (8), (20), (500);
insert into vals values (1), (2), (3), (4), (5), (6), (7), (8), (9), (100);

Output from above query:

1   1
1   2
1   3
4   4
4   5
4   6
4   7
8   8
8   9
20  100

Answer 2

Credit to RhodiumToad on #postgresql

SELECT  * 
FROM    vals v 
JOIN    (select since
              , lead(since) over (order by since) as "end" 
         from intervals) s 
         ON ( v.val >= s.since 
             AND ((v.val >= s."end") IS NOT TRUE)
            )
;