SQL: Merging ranges. Left join?

[BruceBrad]

I'm trying to merge two files which have start and end dates on each record. I want an output file which has all combinations of date spells. Where one file does not have data for a given range, I want to return missing values for its variables.

I almost have this working, but my code doesn't return the desired missing values. (The last record in the desired file in the code). Do I need to make this a left join? (I'm not very familiar with SQL).

data data1;
input start end v1;
if _n_>1 and start^=lag(end)+1 then error;
datalines;
1 2 1.12
3 4 1.34
5 5 1.55
6 7 1.67
8 8 1.88
;
run;

data data2;
input start end v2;
if _n_>1 and start^=lag(end)+1 then error;
datalines;
1 2 2.12
3 3 2.33
4 4 2.44
5 7 2.57
;
run;

* Desired merged file;
data desired;
input start end v1 v2;
datalines;
1 2 1.12 2.12
3 3 1.34 2.23
4 4 1.34 2.44
5 5 1.55 2.57
6 7 1.67 2.57
8 8 1.88  .
;
run;

* get list of time spells;
data long1;
set data1 data2;
keep start end;
run;
proc means data=long1 (keep=end) noprint nway;
  class end;
  output out=long2 (drop=_:) ;
run;
data spells;
set long2;
start = lag(end)+1;
if start=. then start=1;
run;

proc sql;
   CREATE TABLE merged AS
   SELECT s.start, s.end, v1, v2 
   FROM spells as s, data1 as d1, data2 as d2
   where d1.start <= s.start and d1.end >= s.end and
         d2.start <= s.start and d2.end >= s.end ;
   ;
QUIT ;

[BruceBrad · Accepted solution]

I think I've worked this out. Pretty straightforward once I read up on left joins. Required code is below (also there is one error in the 'desired' file. 2.23 should be 2.33).

proc sql;
   CREATE TABLE merged AS
   SELECT s.start, s.end, v1, v2 
     FROM spells as s 
     LEFT JOIN data1 as d1 
     ON d1.start <= s.start and d1.end >= s.end
     LEFT JOIN data2 as d2
     ON d2.start <= s.start and d2.end >= s.end ;
QUIT ;