How to combine natural join and left outer join in SQL - sql

Question

I have 3 tables students, enrollments, and classes. I want to list every student and the department code of each class they have taken (department code is in classes) including the students who have not taken any classes (department code would be blank in this case). This is necessary because of future computations that will be done.
Here is how I've thought about this. The following query does a left join of students and enrollments so that I include students who have not enrolled in any classes
select s.B#, s.firstname, e.classid
from students s, enrollments e
where s.B#=e.B#(+)
This query works as expected.
Next, I join enrollments and classes so that I can see the corresponding dept_code for each enrollment:
select B#, dept_code
from enrollments natural join classes
This is also working as expected.
When I try to bring the two together though I run into problems
select s.B#, s.firstname, dept_code
from students s, (enrollments e natural join classes)
where s.B# = e.B#(+)
Attempting to run this query I get the error:
ORA-25156: old style outer join (+) cannot be used with ANSI joins
Can anyone please explain what's going on here?

Answer 1

You will be better off using explicit JOIN instead of old-style comma-separated joins with where clause. Same goes here for (+).
Write your query like that instead:
select s.B#, s.firstname, dept_code
from students s
left join (
select * -- if there are same names of columns in both tables, specify them explicitly
from enrollments e
join classes c on e.? = c.? -- specify here your natural join columns
) e on s.B# = e.B#
Btw, the error tells you exactly that you can't combine old style joins with ANSI joins. Probably time to switch to the newer format which exists for a long time now.

Answer 2

the brackets are not really needed, I put them there just to emphasize the structure
select s.B#, s.firstname, c.dept_code
from students s
left join ( enrollments e
join classes c
on c.classId = e.classId
)
on s.B# = e.B3
;
Might as well be written like this (only a different format)
select s.B#, s.firstname, c.dept_code
from students s
left join enrollments e
join classes c
on c.classId = e.classId
on s.B# = e.B3
;
P.s.
If you have doubt about this syntax, you might want to try this:
create table t1 (i int);
create table t2 (i int);
create table t3 (i int);
create table t4 (i int);
create table t5 (i int);
insert into t1 (i) values (1);
insert into t2 (i) values (1);
insert into t3 (i) values (1);
insert into t4 (i) values (1);
insert into t5 (i) values (1);
select *
from t1
join t2
join t3
join t4
join t5
on t5.i=t4.i
on t4.i=t3.i
on t3.i=t2.i
on t2.i=t1.i
;

Answer 3

Your query should look something like this:
select s.B#, s.firstname, c.dept_code
from students s left join
enrollments e
on s.B# = e.B3 left join
classes c
on e.classId = b.classId; -- I am guessing what the `JOIN` column is
In other words, you need two left joins to keep all the students. Otherwise, the on clause might filter rows out.
I also strongly encourage you to avoid and un-learn NATURAL JOIN. It is just a bug waiting to happen in code. It matches columns only based on their names -- not even taking into account declared foreign key relationships.
It hides the columns being joins, which makes debugging and understanding queries much more difficult. For instance, because tables I create almost always have columns such as CreatedAt and CreatedBy, it can't be used.
EDIT: (For lgrade)
You would put the condition in the ON clause:
select s.B#, s.firstname, c.dept_code
from students s left join
enrollments e
on s.B# = e.B3 and e.lgrade is not null left join
classes c
on e.classId = b.classId; -- I am guessing what the `JOIN` column is

Answer 4

I strongly encourage you to avoid and un-learn outer join because it produces nulls. Having nulls is just a bug waiting to happen in code. Outer join isn't really a join at all, rather a 'unnatural' union, using nulls to force things together.
Suggested alternative approach: do the union yourself using an appropriate default value:
select B#, firstname, dept_code
from students
natural join enrollments
natural join classes
union
select B#, firstname, '{{NONE}}' as dept_code
from students
where B# not in ( select B# from enrollments );